Interview preparation: the questions people underprepare for

Candidates prepare for the technical questions and for "tell me about yourself". The ones that go badly are usually the behavioural ones — a time you disagreed with a decision, a time you got something wrong, a time you had to say no to a supervisor. These need a specific example with an outcome, and they are hard to invent under pressure. The preparation that works is writing down five or six real situations in advance and knowing them well enough to reshape for whatever is asked. Which question has caught you out, either side of the table?