I Win

Isn't this supposed to be "the most wonderful time of the year"? So why am I feeling like shit?
My first semester back at school is almost done. Just when I thought it had gone off without a hitch, I get an email from one of my teachers saying that he has no record of my taking one of the exams. WTF? Seriously? I said that I don't keep copies of papers or tests that have been graded and returned. He responded that the only option was for me to re-take it, and advised that in the future I should keep all of my papers until the end of the semester "to be covered in such an eventuality."
How about no? Hmm? Ya like that? NO.
So I argued it. Said it was unfair, and that the same could be said about professors keeping papers until the grades are recorded. Ultimately I agreed to take it again if I had to, but made it be known that I found it quite frustrating, especially when I'm preparing for finals.
Know what he did? Sent me an email saying he'll take me at my word, maybe he's made a mistake, and I don't have to re-take it.

I win.