The unhappy path is a feature
Design for invalid input, repeated actions and unavailable services.
15 minutes
Name the failure classes
An invalid input needs correction. An expired session needs sign-in. A temporary upstream failure may permit retry. A permission failure should not be retried as if it were a network blip.
Verify the consequence
For each failure, decide what data remains, what the user sees and whether another attempt is safe. Keep prior input when useful and avoid showing stale information as if it were freshly confirmed.