I teach an expository writing class at a fairly prestigious university on the East Coast. I've luckily experienced less after-the-fact grade-begging than some of my colleagues, which was a relief for the first few semesters I taught because I really hadn't yet felt confident in my own ability to put a solid letter grade on a piece of writing, and I might have caved.
A couple semesters ago, the confidence had solidified, and I was grading the 8-10-page essay of a student we'll call R. It was about 2:00 in the morning, and her essay was terrible - her thesis was basically that "different religions view science differently." That's not a thesis, as one can argue it in about one sentence, not 8-10 pages, and we had discussed in class many times what a thesis ought to look like.
While reading, I encountered a few passages that seemed to deviate from her normal writing style. Ordinarily I'd Google anything suspicious, but it was 2:00 AM, I didn't have a computer nearby, I wanted to hand back this essay the next morning, and I felt the paper's quality was going to sink it anyway. I ended up being slightly kind and giving the paper a "C," reserving D's and F's for students whose essays lacked effort rather than persuasiveness. After handing the paper back, R - who had otherwise been very pleasant - sent me an e-mail in a very officious tone, requesting a meeting to discuss the "C" she claimed not to deserve. I agreed to the dreaded meeting, but before meeting with her, I asked her to e-mail me a clean copy of the paper. After it arrived, I went to Google. Two could play at this game.
Sure enough, the passages I had suspected were indeed lifted from online sources. In fact, about a quarter of her paper could be found verbatim elsewhere. It killed me that she had the nerve to steal other people's writing and beg for a higher grade, but I still wanted to make sure she understood what was wrong with her thesis. So I printed a copy of everything she stole, highlighted the relevant sections, and kept it in reserve. The meeting began exactly as I thought it would. She bitched and moaned and insisted that her thesis was valid, since it's what she did in high school and no one ever reprimanded her then. She was perfectly willing to accept my definition of a thesis, but she refused to accept that hers did not meet the standard.
"It's a research paper," she kept saying, as though that was a fact I had overlooked. Finally, after about half an hour, I gave up. She wasn't going to budge. So I whipped out my ace in the hole. "Okay," I said, "you got me. You're right. You don't deserve a 'C.' You deserve an 'F.'"
Then I showed her the evidence of plagiarism, and it felt very, very good. The unfortunate epilogue that accompanies what would otherwise be a tale of sweet revenge is that she spent the next hour and a half crying and having me assure her that she'd still get into medical school someday. Never drop the plagiarism bomb on a student without bringing a box of tissues. But the "F" stuck.