The thing that triggered this was hearing Nirvana's "Lithium" on the MP3 the other day (not a hilarious event on its own, normally). Was at a 21st, a few years after this had come out. The guest of honour was 6"4, football type and as expected for that sort of event, had quite a few coldies on the night.
Someone cranks up "Lithium" and everyone is getting into it. The birthday boy the most of all. Is standing in a doorway when the chorus really starts getting loud. Forgetting where he is, he jumps vertically (he was a ruck sort and had a pretty fair spring on him) with full enthusiasm.
I was standing 20 feet away and heard the crack of his head on the lintel as if it was next to me. Note - he was absoutely fine, no ill effects, but the whole party just dissolved into laughter. Everytime I hear that song now, it brings a smile to the face.