This happened to me freshman year in band class.
Something I do is I change my jeans every other day. When I take my pants off, I take off my underwear at the same time. So one day I put my pants on, forgetting to take yesterday's underwear out of my jeans. I didn't realize it it until I got to band. I was walking, and I felt something fall out of my pant leg. I thought it was just my imagination. After I got my sax out of the storage room, everyone starts laughing. There's a thong on the floor and no one knows whose it is. I finally owned up to it, saying that my laundry got mixed up with my stepmom's.