What have you shamed of your own actions?

I wasn't really ashamed of this, I was just sort of embarassed.

I was dating a girl in my sophomore year of highschool. The two of us had been together for six or seven months, and we'd long-since gotten to the point of being physically involved with one-another.

So, one day, I'm walking into the cafeteria for lunch and I see my girlfriend standing with her back turned to the door, talking to her friends. I decide to be a jackass and walk up and grab her ass while she's got her back turned. So, I do, and I she lets out this horrified yelp. Then she turns around and I see it's not my fucking girlfriend. It's one of her best friends. My girlfriend was standing ten feet away and watched the whole thing.

The reason it happened was because, from the back, the two of them look nearly identical. Same color hair, same length, same body-type. On top of that, they were both wearing almost the exact same thing when they showed up at school that day.

I just felt like a major dumbass because my girlfriend was actually standing right in plain view when I did it, and she just thought it was fucking hysterical. She kept recounting how fucking hilarious it was to watch me trying to be stealthy and devious about it while she stood there not ten feet away and watched the whole thing.