As to the most embarrassed I think I've ever been, there was one time I was at the beach and I saw my sister in the water, so I went to talk to her and when I got there for some reason I didn't recognize her anymore. I thought I'd mistaken some stranger for my sister so I stuttered out some made-up explanation for what I was doing there. It really was my sister though! I can't even remember what she said but I felt really stupid. I doubt she even remembers this. Then there's those times when I was hitting puberty and my voice/body were pretty out of control and people noticed. Ugh. How embarrassing.
Once in high school, I was waiting outside the bathroom for my girlfriend when one of her female friends, a mutual friend of ours, walked up to me and happily went, "Stephen!" put her hands on my shoulders and KNEED ME IN THE FUCKING BALLS. I dropped like a sack of flour. She walked gleefully away. It was a joke! My girlfriend comes out of the bathroom and is like, "What are you doing? Let's go!"