When somebody says "behind you" you turn around and start randomly attacking anyone nearby.
When you mistake a saw for a violin.
Whenever you are wearing latex gloves you stretch them out and snap them while saying, "that wasn't medicine."
Whenever you finish having a doctor's appointment and you say "Thank you, Herr Doctor"
Whenever you try to drink out of a broken bottle. (Gotta love the demoman)
Whenever you try to put a piece of cardboard over your face when there is a cigarette sticking out of your mouth.
Whenever you look at machinery and hallucinate by seeing the white outline of a sapper.
I'll try to come up with more.