During my university days, we had a basic programming quiz. One of the questions is to "write a program that will determine if a number is even or not".

An annoying seatmate asked me silently if his answer is correct. Then I saw his window:

=========================
> Enter an even number: 10
> The number is even.
=========================

I told him it's correct.

After the test his answer is marked as wrong.

"You told me it's correct!", he said to me.

I approached the professor, and told him that his answer is correct.

"What if I enter 3?", professor said.

I told him, "User Error".