Here's mine.
I was on a school trip when I was 12, and I got up on the first morning and went down to the breakfast hall. My teacher (let's call him Mr. Brown) was looking at me really, really weirdly and all the other teachers were avoiding eye contact. It turns out I had been sleep-walking in the night, and had gone into the teacher's dorm (they all shared), climbed into Mr. Brown's bed with him and said 'move up, you're hogging the duvet!' Humiliating! And I can't remember a thing about it...but I think that makes it even more embarrassing.
What's yours?