There was once a lecture about statistical physics, and I had no idea what the lecturer was explaining. I was merely sitting there, staring at the blackboard, wondering why I'm sitting there, staring at the blackboard. It seems that there was some other student who wasn't understanding anything either, and he started laughing in the middle of the lecture. I cannot be sure of why precisely he started laughing. Perhaps he believed that it was quite common for students in that lecture room to stare at the blackboard without understanding anything, and he then found the entire situation highly ridiculous? Whatever the reason was, he started laughing, which naturally draw attention from everybody else, including the lecturer. Then he got embarrassed, but could not stop laughing, and eventually he escaped out from the lecture room so that the lecture could go on.
IMO it was funny incident.