I have too many to count, can't really judge on which one is the absolute most embarrassing, but I'll share the first couple that came to mind.
In speech class back in high school, we had a unit where each person could choose to do either an informational speech or a comedy sketch. I and two other people chose to do comedy, but those two people ended up changing their minds at the last second, leaving me the only one "with the guts to do so." Turned out it was a big mistake. The only reason I chose comedy was because I was tired of informational essays and speeches and wanted to make it based on the fictional story I was working on at the time. It didn't translate as well as I hoped, and I'm a terrible actor, especially due to my speech issues and mind spinning everywhere when I'm on the spot. It was so humiliating, I scrapped the story entirely because I couldn't separate it from that blunder of a performance.
In freshman year of high school, I visited a friend's house and drank a lot of Pepsi. Hyperactive mood + lots of Pepsi = a spastic moron. It was so bad her parents banned me from their house.
There was another time where I got so excited I danced and sang the Barney song in the cafeteria in middle school.
A lot of my embarrassing moments come from when I got hyperactive, now that I think about it. I'm usually very calm and quiet around people, but when I get excited and manage to ignore what other people think for a moment, I become a brainless weirdo. It's a MASSIVE leap to anyone used to just seeing my quiet side, so it's even worse. I'm much better at controlling it now, but it still happens.
Same question.