First Day of Classes...
One thing that's always worried me, but luckily that's never happened (yet).
But, on the first day of classes, in my Advanced French Conversation course, that happened to some poor guy. Before class started, about half the students were speaking to each other in English, and the other half in French. Knowing that there's a tiny (almost non-existent) Francophone population here, you think that might be your first clue that you're in a French class. Maybe.
Then the prof comes in, a bit late, and starts speaking to the class in French. He was introducing himself, and a bit about the course, when this guy walks up to the front of the class to leave. Keep in mind that this is about 5 minutes into class. The prof asked him, en francais, if he was in the right class. He just looked blankly at the prof. Then the prof repeated the question, and the guy said, "yah, I don't think this is history." Oh, that would be embarrassing. But all of us in the class got a good laugh out of it!