During the process of grading a final exam, one student's answer to a particular question was very different from my own "correct" answer, but persuasive enough to make me question my own assumptions.
After looking a few things up, I think the student is right. I now have to change my own answer.
Naturally, this was the last exam I graded, so now I get to go back through every single other one and redo the grading on this question. (I won't take points away from students who made the same mistake I did.)
I have to wonder how common this is. I know that every teacher says they learn as much as their students, but I didn't expect to be learning things after the class was already over.