I believe you under-estimate what a good interviewer is trying to do with questions such as these:
Either you've seen the trick before and you get an opportunity to show the interviewer that you're an honest person by telling him you have. Huge plus and the interview can move on to other topics.
Either you haven't and you can demonstrate to the interviewer your analytical skills by dissecting the problem step by step and understanding what the code actually does and how.
Bonus if you can see the potential aliasing problem when used to swap two variables.
Not a trivia question at all.