The most embarrassing thing was back when I was fourteen, and was riding my bike out with some friends.
I had the kind of bike that tended to ejects the chain from its mechanism when I looked at it the wrong way, so I had to ride it extra careful.
Of course, when we were in the city and there was a plastic cup on the floor, which I accidentally hit, I immediately looked down to check what happened, and if my bike chain was alright. It was, but in the two seconds it took me to check, I didn't see what was in front of me, an admittedly bad order of priorities.

Long story short, I slammed into the big-wooden-menu thing in front of a pizzeria and utterly annihilated it, in front of a couple hundred people.
I profusely apologized to the owner, accepted to recompensate him for the property damage, and after paying it back never went back to that place ever again.