Everything worked. I'd wired up useOptimistic on a task list, the checkbox flipped the instant you clicked it, no spinner, no half-second lag, exactly the feeling I was going for. I demoed it to myself a dozen times and moved on.
Then I did the thing you're supposed to do before you actually ship anything: I clicked it stupidly fast, five times in a row, the way an impatient real user actually would.
The checkbox stayed visually fine. The database did not. Five overlapping requests had gone out, each one flipping the same boolean, landing in whatever order the network felt like that day. Nothing crashed. Nothing logged an error. The UI just quietly stopped matching what actually happened on the server, and I had no way of knowing from looking at the screen.
That's the bug that's easy to miss with optimistic UI specifically, because the whole point of optimistic UI is that it looks right immediately. Looking right and being right are not the same claim.
The fix isn't anything specific to useOptimistic