When I was probably 12-13 years old I performed the dance to the Backstreet Boys hit Backstreet's Back (I think is the name of it) in public.
I'd like to say I was coerced into doing it, but I wasn't, I wasn't at all. It was just one of those things where someone had the idea and there was an opportunity, and I don't know. Looking back, it helped me understand the concepts of temporary insanity and mob mentality pretty well. I thought they were going to clap and cheer, I have no idea why. There was just this silence and the knowledge of the mockery which would follow. We finished, I saw the reaction, time itself stopped as the realization of the act I had just performed became real and it broke my ability to be embarassed.
Nothing can top that. I sing loudly along to everything from Manowar to Bon Jovi to Meatloaf and I just don't give a shit now.
Edit: Oh, and to keep the thread going, the song which gives me the most bizarre looks when I sing along is
Cutting Crew - (I just) died in your arms tonight. It's an epic song but I think it's the way the chorus inspires the volume of my voice.