I shared a concern that I had with a student this week that his exercise was a plagiarism. It consisted of two parts, one part was barely in coherent English with many misspelled words, the second (set in a different typeface) was nicely written with only the occasional misspelling.
I couldn't find anything with a search machine, but I suspect it was borrowed from a previous year's course. I tried to confer with the teacher of that course, but she is on vacation this week, so I attempted to communicate with the student.
He erupted into anger - was I calling him a criminal? No, I was curious as to the explanation for the surprising differences. He berated me, insisted there was no difference. This made me analyze the parts: part 1 was half a page and had 18 spelling errors and badly structured sentences. Part 2 was a page and a half, had correctly structured sentences and only 12 errors in total, or 4 per half a page. That's a power of 10 difference.
I asked the student for patience and told him that if the other teacher does not recognize the paper, he will get the normal amount of points for the exercise.
He threw back an email (the question of how reasonable it is to fight with a teacher in a tone like this is beside the point) saying that if I look hard enough I will find plagiarism everywhere I want to. He said it rather nastily, but he does have a point: If you find a lot of plagiarism, do you start (wrongly) thinking that most students are plagiarists and seeing plagiarism everywhere?
Or is plagiarism just so rampart that we absolutely must suspect it everywhere?