When I was a TA a while back, I got an unmemorable but adequate paper, gave it a B-. Student comes to my office, says "I have a concern about my grade. You gave it a B-, but my girlfriend turned it in to another TA and she got a B." I paused for a couple of seconds, maintained my composure, and calmly said "that does sound like a problem. If you can bring me your paper and your girlfriend's paper, hopefully we can sort this out". So he does, and we turned them both in for plagiarism.
Actually, that wasn't a frustration. It was awesome.
You really should read some of the others on there.