My class had an unofficial rule that no questions were to be asked in the last ten minutes of lecture to prevent the lecturer from eating into our break time. Most questions are asinine, anyways, and asked by people who either already know the answer or could just as easily look it up themselves or ask the professor after lecture.
Besides, since most lectures are just glorified trivial pursuit, even if the professor manages to clear up some confusion over some insignificant topic it is still an insignificant fact which in the great scheme of things means nothing. I'd rather have a full ten minute break then have a meaningless part of the TCA cycle clarified.
It can get even worse on rotations. Never, ever, ask a question. If you really want to know you can make a note and look it up later (what I did). Asking a question of your attending while rounding opens you up to all sorts of unpleasantness, from pimping to being asked to prepare a short, five minute presentation for the team. I tell all the third years who I rotate with that it is best to stand mute wearing a look of polite interest.
Funny story. Some resident not even on my service tried to pimp me the other day on some subject of which I had an imperfect grasp. After contemplating venturing a guess I just said, "I don't know but if you want," pointing to my Scutmonkey Clinical Reference Manual, "I can look it up for you."
This made him irate. I volunteered that I don't memorize trivia which made him even madder. He kept firing questions at me to each of which I replied, "I haven't a clue." Finally he threatened me with some kind of second-hand bad evaluation (he knew a guy who knew a guy who knew my attending). I couldn't help but laugh.
First of all, and I only realized this recently, you don't have to kiss anybody's ass or accept any mistreatment at anybody's hands. Let's suppose that some resident is treating you like dirt. I guarantee that if you take him aside and threaten to report his harrasment he will back off. We are all men (and women) and it is pretty hard for a resident, when called to the carpet, to explain why he felt it necessary to call you a name or treat you with anything less than respect.
Fortunantly I have had a lot of good residents and the few times they have zinged me with some sarcasm I richly deserved it for doing something really stupid.
Second, I'm going to graduate in three months, fourth year is all pass/fail, and I don't believe anybody was ever failed for a rotation because they didn't know the answer to a pimp question.
Let me just say, however, that most residents and attendings pimp you as a teaching tool so you shouldn't go around with a flippant attitude. "I don't know" is acceptable but most good residents and attendings at least want you to think about it for a second or two and offer a reasonable answer even if you are wrong.
gotta love the students that boost your grade 
