When I interviewed at Google in 2006, one of the interviewers asked me to write code for solving a particular problem. I replied along the lines of "ok I need to think about this, it's not obvious how to solve it" and started thinking. Time ticked away. Five minutes. Ten minutes. Fifteen minutes. The interviewer started reminding me that he needed me to write some code and if there wasn't anything for him to copy down into his interview notes I wasn't going to pass the interview. (Why interviewers were all
transcribing the whiteboard rather than taking photos I have no clue.)
Eventually I ran out of time, having written no code on the whiteboard. I asked my interviewer "ok so what is the algorithm for solving this" and he admitted that he had no idea.
At the time, I thought he just meant that he had no idea what the solution was; but in hindsight I wonder if the real test was to see if I would crumble under pressure and write code I knew didn't work.
(I got a job offer, but turned it down to start Tarsnap instead.)