I used to go to evening classes with a friend to learn Spanish and the teacher was rather strict. The class was held in a school and one evening we were late so we ran down the corridor. My friend was in front of me and just as we got there, she slipped and fell over right outside the classroom door.
OMG it was so funny, the panic of being late and her going flying just set me off but we couldn't laugh because the class would hear us and the door was one that was half solid with a pane in the top so we had to hide, crouching under under the window and stifling our laughter. It was so funny, it still makes me laugh now.