Puzzles can be so easily misused. If the interviewer(s) all know the answer to the puzzle they'll have trouble being objective when it comes to measuring the responses of an interviewee who hasn't seen the puzzle before.

Worse still is that an underinformed interviewer might simply tune out until the candidate returns an answer to the puzzle rather than actively participating in the candidate's problem-solving process. Two engineers and a whiteboard is how all the best work gets done, right?