diff --git a/knowledge.md b/knowledge.md
index 9443e7b..e83de43 100644
--- a/knowledge.md
+++ b/knowledge.md
@@ -13,6 +13,10 @@ Use `pnpm verify` to check that code changes are valid.
   - Pending: Can edit, complete, uncle, copy
   - Complete: Can mark incomplete, copy
   - Expired: Can copy
+- Completing tasks requires careful handling:
+  - Marking tasks complete is a critical action
+  - Reversing completed tasks requires support intervention
+  - UI should prevent accidental completion
 
 ## Future Improvements
 
diff --git a/src/components/task.svelte b/src/components/task.svelte
index e4705a2..4c73bc7 100644
--- a/src/components/task.svelte
+++ b/src/components/task.svelte
@@ -21,6 +21,18 @@
 
 	async function toggleComplete() {
 		if (!task.id) return;
+
+		const taskDue = new Date(task.due);
+		const now = new Date();
+		const isPastDue = taskDue < now;
+
+		if (isPastDue && !task.complete) {
+			const confirmed = confirm(
+				'This task is past due. Marking it complete will require contacting support to undo. Continue?'
+			);
+			if (!confirmed) return;
+		}
+
 		await updateTask(task.id, { complete: !task.complete });
 		task.complete = !task.complete;
 		task.status = task.complete ? 'complete' : 'pending';