Race conditions in Next.js 16 useOptimistic

Implementing useOptimistic for a snappy task list in Next.js 16 felt like a win until I actually tested the edge cases of human behavior, because as it turns out, "snappy" is just a polite way of saying "highly susceptible to race conditions" if you aren't careful. I was testing a checkbox toggle and decided to hammer it with rapid-fire clicks just to see what would happen, and while the UI looked perfect—thanks to React's transition logic—the backend state became a complete disaster because five simultaneous requests hit the server and resolved out of order. You see these tutorials where people say "useOptimistic makes the UI feel real," but they rarely mention that looking right and actually being right are two different things; if your database ends up in a state that contradicts the client-side view, you've built a lie. I've seen people suggest that you need complex rollback logic in a catch block to "undo" the state, but that’s actually a bit of a misunderstanding of how the hook works under the hood. React is supposed to handle the revert automatically by dropping the optimistic layer once the transition settles, so if the server rejects the change, the UI should just snap back to the actual server state naturally. The real problem isn't the rollback; it's the concurrency. I had to implement a pendingId state at the row level to essentially lock the interaction until the transition settles, which prevents those redundant writes from ever hitting the server in the first place. The one area where the "magic" fails is when you're dealing with brand new items using temporary client-side IDs, because there is no legitimate previous state to revert to if that database insert fails, meaning you still have to write manual cleanup logic for those specific cases. You can't just rely on the happy path and assume the framework will save you from a chaotic user; you have to design for the fact that the network is asynchronous and users are impatient.