These are painful to read, lol.
Mine would have to be in 6th grade. I had prepared really hard for a presentation in front of my history class. I walked up there and started giving my report, but everyone was sort of staring at me. As I talked some kids in front started making jokes about being distracted by a fly. I didn't let myself be thrown off though(I had worked hard on this) and kept going, but now people were looking at me and laughing. Then a merciful soul finally told me, "Your fly is down".
I had been talking a good minute with my zipper wide open. I was beyond embarrassed and just walked back to my seat with everyone laughing at me. It's still a sore spot in my memory.