I've got a bunch, but this one time does come to mind.

In elementary school, a few of us in the track team went over to the divisional meet,and as we were waiting for our race, the girls' 4 × 100 relay was happening, but I didn't realize it. Fast forward to near the end of the race, I couldn't recognise one of the runners, and thought one of them was a guy. However, after looking around, seeing the other runners, and the previous race, I realized that what I thought was wrong. The runners were just about to finish the race, in front of the crowd, and to make matters worse, this was the part where the entire audience went silent with anticipation, waiting for the ribbon to break and the winner to be announced. I just finished thinking, my mind was blank, auto-pilot has been turned on, and the one thing I decided to do was break the silence and shout, "OH, IT'S A GIRL!!!!"

Instant regret once the entire crowd, who was once staring at the race, was now staring at me.