diff --git a/projects/common/src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorBlockEntity.java b/projects/common/src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorBlockEntity.java
index fbd451bf5..a08feac5a 100644
--- a/projects/common/src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorBlockEntity.java
+++ b/projects/common/src/main/java/dan200/computercraft/shared/peripheral/monitor/MonitorBlockEntity.java
@@ -157,7 +157,6 @@ public class MonitorBlockEntity extends BlockEntity {
         if (xIndex == 0 && yIndex == 0) {
             // If we're the origin, set up the new monitor
             serverMonitor = new ServerMonitor(advanced, this);
-            serverMonitor.rebuild();
 
             // And propagate it to child monitors
             for (var x = 0; x < width; x++) {
@@ -178,6 +177,11 @@ public class MonitorBlockEntity extends BlockEntity {
         }
     }
 
+    private void createServerTerminal() {
+        var monitor = createServerMonitor();
+        if (monitor != null && monitor.getTerminal() == null) monitor.rebuild();
+    }
+
     @Nullable
     public ClientMonitor getClientMonitor() {
         if (clientMonitor != null) return clientMonitor;
@@ -377,6 +381,8 @@ public class MonitorBlockEntity extends BlockEntity {
                 BlockEntityHelpers.updateBlock(monitor);
             }
         }
+
+        assertInvariant();
     }
 
     void updateNeighborsDeferred() {
@@ -487,9 +493,10 @@ public class MonitorBlockEntity extends BlockEntity {
     }
 
     public IPeripheral peripheral() {
-        createServerMonitor();
-        if (peripheral != null) return peripheral;
-        return peripheral = new MonitorPeripheral(this);
+        createServerTerminal();
+        var peripheral = this.peripheral != null ? this.peripheral : (this.peripheral = new MonitorPeripheral(this));
+        assertInvariant();
+        return peripheral;
     }
 
     void addComputer(IComputerAccess computer) {
@@ -528,4 +535,85 @@ public class MonitorBlockEntity extends BlockEntity {
             Math.max(startPos.getZ(), endPos.getZ()) + 1
         );
     }
+
+    /**
+     * Assert all {@linkplain #checkInvariants() monitor invariants} hold.
+     */
+    private void assertInvariant() {
+        assert checkInvariants() : "Monitor invariants failed. See logs.";
+    }
+
+    /**
+     * Check various invariants about this monitor multiblock. This is only called when assertions are enabled, so
+     * will be skipped outside of tests.
+     *
+     * @return Whether all invariants passed.
+     */
+    private boolean checkInvariants() {
+        LOG.debug("Checking monitor invariants at {}", getBlockPos());
+
+        var okay = true;
+
+        if (width <= 0 || height <= 0) {
+            okay = false;
+            LOG.error("Monitor {} has non-positive of {}x{}", getBlockPos(), width, height);
+        }
+
+        var hasPeripheral = false;
+        var origin = getOrigin().getMonitor();
+        var serverMonitor = origin != null ? origin.serverMonitor : this.serverMonitor;
+        for (var x = 0; x < width; x++) {
+            for (var y = 0; y < height; y++) {
+                var monitor = getLoadedMonitor(x, y).getMonitor();
+                if (monitor == null) continue;
+
+                hasPeripheral |= monitor.peripheral != null;
+
+                if (monitor.serverMonitor != null && monitor.serverMonitor != serverMonitor) {
+                    okay = false;
+                    LOG.error(
+                        "Monitor {} expected to be have serverMonitor={}, but was {}",
+                        monitor.getBlockPos(), serverMonitor, monitor.serverMonitor
+                    );
+                }
+
+                if (monitor.xIndex != x || monitor.yIndex != y) {
+                    okay = false;
+                    LOG.error(
+                        "Monitor {} expected to be at {},{}, but believes it is {},{}",
+                        monitor.getBlockPos(), x, y, monitor.xIndex, monitor.yIndex
+                    );
+                }
+
+                if (monitor.width != width || monitor.height != height) {
+                    okay = false;
+                    LOG.error(
+                        "Monitor {} expected to be size {},{}, but believes it is {},{}",
+                        monitor.getBlockPos(), width, height, monitor.width, monitor.height
+                    );
+                }
+
+                var expectedState = getBlockState().setValue(MonitorBlock.STATE, MonitorEdgeState.fromConnections(
+                    y < height - 1, y > 0, x > 0, x < width - 1
+                ));
+                if (monitor.getBlockState() != expectedState) {
+                    okay = false;
+                    LOG.error(
+                        "Monitor {} expected to have state {}, but has state {}",
+                        monitor.getBlockState(), expectedState, monitor.getBlockState()
+                    );
+                }
+            }
+        }
+
+        if (hasPeripheral != (serverMonitor != null && serverMonitor.getTerminal() != null)) {
+            okay = false;
+            LOG.error(
+                "Peripheral is {}, but serverMonitor={} and serverMonitor.terminal={}",
+                hasPeripheral, serverMonitor, serverMonitor == null ? null : serverMonitor.getTerminal()
+            );
+        }
+
+        return okay;
+    }
 }
diff --git a/projects/common/src/testMod/kotlin/dan200/computercraft/gametest/Monitor_Test.kt b/projects/common/src/testMod/kotlin/dan200/computercraft/gametest/Monitor_Test.kt
index 4e9c54bed..2433a5893 100644
--- a/projects/common/src/testMod/kotlin/dan200/computercraft/gametest/Monitor_Test.kt
+++ b/projects/common/src/testMod/kotlin/dan200/computercraft/gametest/Monitor_Test.kt
@@ -17,7 +17,10 @@ import net.minecraft.gametest.framework.GameTestGenerator
 import net.minecraft.gametest.framework.GameTestHelper
 import net.minecraft.gametest.framework.TestFunction
 import net.minecraft.nbt.CompoundTag
+import net.minecraft.world.entity.EntityType
+import net.minecraft.world.item.ItemStack
 import net.minecraft.world.level.block.Blocks
+import org.junit.jupiter.api.Assertions.*
 import java.util.*
 
 class Monitor_Test {
@@ -61,6 +64,45 @@ class Monitor_Test {
         }
     }
 
+    /**
+     * When a monitor is destroyed and then replaced, the terminal is recreated.
+     */
+    @GameTest
+    fun Creates_terminal(helper: GameTestHelper) = helper.sequence {
+        fun monitorAt(x: Int) =
+            helper.getBlockEntity(BlockPos(x, 2, 2), ModRegistry.BlockEntities.MONITOR_ADVANCED.get())
+
+        thenExecute {
+            for (i in 1..3) {
+                assertNull(monitorAt(i).cachedServerMonitor, "Monitor $i starts with no ServerMonitor")
+            }
+
+            monitorAt(2).peripheral()
+            assertNotNull(monitorAt(1).cachedServerMonitor?.terminal, "Creating a peripheral creates a terminal")
+
+            // Then remove the middle monitor and check it splits into two.
+            helper.setBlock(BlockPos(2, 2, 2), Blocks.AIR.defaultBlockState())
+
+            assertNotNull(monitorAt(3).cachedServerMonitor, "Origin retains its monitor")
+            assertNull(monitorAt(3).cachedServerMonitor!!.terminal, "Origin deletes the terminal")
+            assertNotEquals(monitorAt(1).cachedServerMonitor, monitorAt(3).cachedServerMonitor, "Monitors are different")
+
+            // Then set the monitor, check it rejoins and recreates the terminal.
+            val pos = BlockPos(2, 2, 2)
+            helper.setBlock(pos, ModRegistry.Blocks.MONITOR_ADVANCED.get())
+            ModRegistry.Blocks.MONITOR_ADVANCED.get().setPlacedBy(
+                helper.level,
+                helper.absolutePos(pos),
+                helper.getBlockState(pos),
+                EntityType.COW.create(helper.level),
+                ItemStack.EMPTY,
+            )
+            monitorAt(2).peripheral()
+
+            assertNotNull(monitorAt(1).cachedServerMonitor?.terminal, "Recreates the terminal")
+        }
+    }
+
     /**
      * Test monitors render correctly
      */
diff --git a/projects/common/src/testMod/resources/data/cctest/structures/monitor_test.creates_terminal.snbt b/projects/common/src/testMod/resources/data/cctest/structures/monitor_test.creates_terminal.snbt
new file mode 100644
index 000000000..83f4fcea8
--- /dev/null
+++ b/projects/common/src/testMod/resources/data/cctest/structures/monitor_test.creates_terminal.snbt
@@ -0,0 +1,139 @@
+{
+    DataVersion: 3465,
+    size: [5, 5, 5],
+    data: [
+        {pos: [0, 0, 0], state: "minecraft:polished_andesite"},
+        {pos: [0, 0, 1], state: "minecraft:polished_andesite"},
+        {pos: [0, 0, 2], state: "minecraft:polished_andesite"},
+        {pos: [0, 0, 3], state: "minecraft:polished_andesite"},
+        {pos: [0, 0, 4], state: "minecraft:polished_andesite"},
+        {pos: [1, 0, 0], state: "minecraft:polished_andesite"},
+        {pos: [1, 0, 1], state: "minecraft:polished_andesite"},
+        {pos: [1, 0, 2], state: "minecraft:polished_andesite"},
+        {pos: [1, 0, 3], state: "minecraft:polished_andesite"},
+        {pos: [1, 0, 4], state: "minecraft:polished_andesite"},
+        {pos: [2, 0, 0], state: "minecraft:polished_andesite"},
+        {pos: [2, 0, 1], state: "minecraft:polished_andesite"},
+        {pos: [2, 0, 2], state: "minecraft:polished_andesite"},
+        {pos: [2, 0, 3], state: "minecraft:polished_andesite"},
+        {pos: [2, 0, 4], state: "minecraft:polished_andesite"},
+        {pos: [3, 0, 0], state: "minecraft:polished_andesite"},
+        {pos: [3, 0, 1], state: "minecraft:polished_andesite"},
+        {pos: [3, 0, 2], state: "minecraft:polished_andesite"},
+        {pos: [3, 0, 3], state: "minecraft:polished_andesite"},
+        {pos: [3, 0, 4], state: "minecraft:polished_andesite"},
+        {pos: [4, 0, 0], state: "minecraft:polished_andesite"},
+        {pos: [4, 0, 1], state: "minecraft:polished_andesite"},
+        {pos: [4, 0, 2], state: "minecraft:polished_andesite"},
+        {pos: [4, 0, 3], state: "minecraft:polished_andesite"},
+        {pos: [4, 0, 4], state: "minecraft:polished_andesite"},
+        {pos: [0, 1, 0], state: "minecraft:air"},
+        {pos: [0, 1, 1], state: "minecraft:air"},
+        {pos: [0, 1, 2], state: "minecraft:air"},
+        {pos: [0, 1, 3], state: "minecraft:air"},
+        {pos: [0, 1, 4], state: "minecraft:air"},
+        {pos: [1, 1, 0], state: "minecraft:air"},
+        {pos: [1, 1, 1], state: "minecraft:air"},
+        {pos: [1, 1, 2], state: "computercraft:monitor_advanced{facing:north,orientation:north,state:l}", nbt: {Height: 1, Width: 3, XIndex: 2, YIndex: 0, id: "computercraft:monitor_advanced"}},
+        {pos: [1, 1, 3], state: "minecraft:air"},
+        {pos: [1, 1, 4], state: "minecraft:air"},
+        {pos: [2, 1, 0], state: "minecraft:air"},
+        {pos: [2, 1, 1], state: "minecraft:air"},
+        {pos: [2, 1, 2], state: "computercraft:monitor_advanced{facing:north,orientation:north,state:lr}", nbt: {Height: 1, Width: 3, XIndex: 1, YIndex: 0, id: "computercraft:monitor_advanced"}},
+        {pos: [2, 1, 3], state: "minecraft:air"},
+        {pos: [2, 1, 4], state: "minecraft:air"},
+        {pos: [3, 1, 0], state: "minecraft:air"},
+        {pos: [3, 1, 1], state: "minecraft:air"},
+        {pos: [3, 1, 2], state: "computercraft:monitor_advanced{facing:north,orientation:north,state:r}", nbt: {Height: 1, Width: 3, XIndex: 0, YIndex: 0, id: "computercraft:monitor_advanced"}},
+        {pos: [3, 1, 3], state: "minecraft:air"},
+        {pos: [3, 1, 4], state: "minecraft:air"},
+        {pos: [4, 1, 0], state: "minecraft:air"},
+        {pos: [4, 1, 1], state: "minecraft:air"},
+        {pos: [4, 1, 2], state: "minecraft:air"},
+        {pos: [4, 1, 3], state: "minecraft:air"},
+        {pos: [4, 1, 4], state: "minecraft:air"},
+        {pos: [0, 3, 0], state: "minecraft:air"},
+        {pos: [0, 2, 1], state: "minecraft:air"},
+        {pos: [0, 2, 2], state: "minecraft:air"},
+        {pos: [0, 2, 3], state: "minecraft:air"},
+        {pos: [0, 2, 4], state: "minecraft:air"},
+        {pos: [1, 2, 0], state: "minecraft:air"},
+        {pos: [1, 2, 1], state: "minecraft:air"},
+        {pos: [1, 2, 2], state: "minecraft:air"},
+        {pos: [1, 2, 3], state: "minecraft:air"},
+        {pos: [1, 2, 4], state: "minecraft:air"},
+        {pos: [2, 2, 0], state: "minecraft:air"},
+        {pos: [2, 2, 1], state: "minecraft:air"},
+        {pos: [2, 2, 2], state: "minecraft:air"},
+        {pos: [2, 2, 3], state: "minecraft:air"},
+        {pos: [2, 2, 4], state: "minecraft:air"},
+        {pos: [3, 2, 0], state: "minecraft:air"},
+        {pos: [3, 2, 1], state: "minecraft:air"},
+        {pos: [3, 2, 2], state: "minecraft:air"},
+        {pos: [3, 2, 3], state: "minecraft:air"},
+        {pos: [3, 2, 4], state: "minecraft:air"},
+        {pos: [4, 2, 0], state: "minecraft:air"},
+        {pos: [4, 2, 1], state: "minecraft:air"},
+        {pos: [4, 2, 2], state: "minecraft:air"},
+        {pos: [4, 2, 3], state: "minecraft:air"},
+        {pos: [4, 2, 4], state: "minecraft:air"},
+        {pos: [0, 3, 0], state: "minecraft:air"},
+        {pos: [0, 3, 1], state: "minecraft:air"},
+        {pos: [0, 3, 2], state: "minecraft:air"},
+        {pos: [0, 3, 3], state: "minecraft:air"},
+        {pos: [0, 3, 4], state: "minecraft:air"},
+        {pos: [1, 3, 0], state: "minecraft:air"},
+        {pos: [1, 3, 1], state: "minecraft:air"},
+        {pos: [1, 3, 2], state: "minecraft:air"},
+        {pos: [1, 3, 3], state: "minecraft:air"},
+        {pos: [1, 3, 4], state: "minecraft:air"},
+        {pos: [2, 3, 0], state: "minecraft:air"},
+        {pos: [2, 3, 1], state: "minecraft:air"},
+        {pos: [2, 3, 2], state: "minecraft:air"},
+        {pos: [2, 3, 3], state: "minecraft:air"},
+        {pos: [2, 3, 4], state: "minecraft:air"},
+        {pos: [3, 3, 0], state: "minecraft:air"},
+        {pos: [3, 3, 1], state: "minecraft:air"},
+        {pos: [3, 3, 2], state: "minecraft:air"},
+        {pos: [3, 3, 3], state: "minecraft:air"},
+        {pos: [3, 3, 4], state: "minecraft:air"},
+        {pos: [4, 3, 0], state: "minecraft:air"},
+        {pos: [4, 3, 1], state: "minecraft:air"},
+        {pos: [4, 3, 2], state: "minecraft:air"},
+        {pos: [4, 3, 3], state: "minecraft:air"},
+        {pos: [4, 3, 4], state: "minecraft:air"},
+        {pos: [0, 4, 0], state: "minecraft:air"},
+        {pos: [0, 4, 1], state: "minecraft:air"},
+        {pos: [0, 4, 2], state: "minecraft:air"},
+        {pos: [0, 4, 3], state: "minecraft:air"},
+        {pos: [0, 4, 4], state: "minecraft:air"},
+        {pos: [1, 4, 0], state: "minecraft:air"},
+        {pos: [1, 4, 1], state: "minecraft:air"},
+        {pos: [1, 4, 2], state: "minecraft:air"},
+        {pos: [1, 4, 3], state: "minecraft:air"},
+        {pos: [1, 4, 4], state: "minecraft:air"},
+        {pos: [2, 4, 0], state: "minecraft:air"},
+        {pos: [2, 4, 1], state: "minecraft:air"},
+        {pos: [2, 4, 2], state: "minecraft:air"},
+        {pos: [2, 4, 3], state: "minecraft:air"},
+        {pos: [2, 4, 4], state: "minecraft:air"},
+        {pos: [3, 4, 0], state: "minecraft:air"},
+        {pos: [3, 4, 1], state: "minecraft:air"},
+        {pos: [3, 4, 2], state: "minecraft:air"},
+        {pos: [3, 4, 3], state: "minecraft:air"},
+        {pos: [3, 4, 4], state: "minecraft:air"},
+        {pos: [4, 4, 0], state: "minecraft:air"},
+        {pos: [4, 4, 1], state: "minecraft:air"},
+        {pos: [4, 4, 2], state: "minecraft:air"},
+        {pos: [4, 4, 3], state: "minecraft:air"},
+        {pos: [4, 4, 4], state: "minecraft:air"}
+    ],
+    entities: [],
+    palette: [
+        "minecraft:polished_andesite",
+        "minecraft:air",
+        "computercraft:monitor_advanced{facing:north,orientation:north,state:l}",
+        "computercraft:monitor_advanced{facing:north,orientation:north,state:lr}",
+        "computercraft:monitor_advanced{facing:north,orientation:north,state:r}"
+    ]
+}
diff --git a/projects/fabric/build.gradle.kts b/projects/fabric/build.gradle.kts
index 3a737ca38..9e81e6a99 100644
--- a/projects/fabric/build.gradle.kts
+++ b/projects/fabric/build.gradle.kts
@@ -150,6 +150,8 @@ loom {
 
             // Load cctest last, so it can override resources. This bypasses Fabric's shuffling of mods
             property("fabric.debug.loadLate", "cctest")
+
+            vmArg("-ea")
         }
 
         val testClient by registering {
diff --git a/projects/forge/build.gradle.kts b/projects/forge/build.gradle.kts
index bb80450e8..ac6882a3e 100644
--- a/projects/forge/build.gradle.kts
+++ b/projects/forge/build.gradle.kts
@@ -102,6 +102,7 @@ minecraft {
             configureForGameTest()
 
             property("forge.logging.console.level", "info")
+            jvmArg("-ea")
         }
     }
 }