Especially when you're trying to help a student and the student argues with you and states that they don't agree with your answer.
I was trying to help this student the other day and he kept on arguing with me, "I think I understand what you're saying but I don't agree with it." Well, I'm sorry, but I didn't make up the f*cking rules!
We were given the answers and I try my best to explain it as much as possible. Don't argue with me if you don't agree with what I say - I'm just stating what is in the answer key because I don't want to give you the wrong instructions on approaching this problem. So really, it's not me that you're disagreeing with - it's the prof.