I was in a calculus class with a guy I had become friends with. The professor was teaching two sections of the class and at the end of the semester he said we could take the final during the assigned time for either of his sections. I wanted to get my final over with early so I took it at the alternate time with the people from his other class.

There was some particularly tricky/difficult problem we had worked on in homework and I studied it a lot before the final because it was what I was most worried about remembering. It wasn't on the final and I breezed through the questions no problem. My friend was taking the final the next day during our normally assigned time. I didn't tell him what was on the final but when he was studying he asked me something about this particularly tricky problem I had been worried about. I just said something like, oh, I wouldn't worry that much about it. The next day he was cursing me and thought I had sabotaged him because that problem was indeed on the final and he didn't do well on it. The professor had made two different finals for the class. I felt bad.