answer
I said that I messed up something in my first week of the job (something very small) and that my boss pulled me up on it. My boss explained my error and i accepted it that I was wrong. And I didn't make that mistake again.
Even if it sounds like they want an example of when you had an argument with your boss, they really don't want to hear that you did.
If all else fails, I'm sure it wont be a problem to say (after a minute or two thinking) "I don't think that I have ever been in that situation"