Ah yes, you do have to calibrate your sense of humour according to the audience. I don't have any memories of screwing up particularly badly myself (though perhaps my brain has erased all evidence of the trauma) but I've been at plenty of talks where the speaker's jokes fell completely flat, and it's really awful. I think spontaneity in humour is very important; jokes can be rather unfunny if you can tell that they were prepared beforehand. Spontaneity in general is a good thing, though; there's nothing worse than a speech that sounds too rehearsed - preparation, like all things, requires a great deal of judgement as to where "too much" lies. I've never freaked out too badly, but I did hear about a friend of a friend who started crying during a debate. She actually finished, crying the entire time.