answer:Yes, this does sound a bit like confirmation bias to me. If there are only two suspicious lines in the whole paper, and the paper is good throughout, then this sounds like a case of convergent evolution. It happens. I just finished grading 64 papers on the same topic. Unsurprisingly, many of the same concerns came up over and over again. Had I bothered to compare, I’m sure there would have been some near-identical sentences in a few of those papers. Am I to assume those students were collaborating with one another in order to cheat? No. Like you say, there are only so many ways to say something. What always worries me is when a particularly good sentence or two stand out from a bad paper. I always put those into Google, often trying a couple variations, and they almost invariably come up in some document that would also rank high in a search for the paper topic. Similarly, a really good paper from a bad student arouses suspicion. But if I get a paper that is good throughout from a good student, I see no reason for surprise. If part of it turns out to be similar to something else, I just conclude that great minds think alike until I have reason to think otherwise. As for the threat—and it’s definitely a threat—it sounds to me like a “don’t challenge me because I can’t back this up” move. I’ve taught for professors who do similar things in order to forestall challenges to grades given by the teaching assistants. I generally don’t like that kind of tactic, but it is particularly disgusting in a situation like this where it is being used to prevent someone from defending herself. It is unprofessional, and any grade change could probably be appealed to a higher power on the grounds that it was retaliatory if the plagiarism charge gets dropped.