Oh yeah I've had the misfortune of giving hundreds of interviews -- mostly programming interviews, but also talking interviews which I consider vastly superior. As well as being on the receiving end of a few.

I've definitely had people do poorly under pressure before. This can be over-complicating the problem, clamming up (surprisingly common), or simply getting too worked up by the interview setting. I hate that because I often think they could have met my rubric in a more relaxed environment.

I've also been on the receiving end of bad interviewers. Don't get me started on HP asking me to implement offsetof in C++... n.b. implementing offsetof in C++ w/o undefined behavior is impossible it has to be a compiler builtin.