It's frustrating when a student plagiarizes, by copying directly from the web.
Its more frustrating when some of it is the student's writing (13 words/55), some paraphrased very closely (12 words/55), and some of it is verbatim (30/55), leaving room for a little nagging doubt.
It's really frustrating when the assignment for which the student submitted a plagiarized response is merely a discussion question exercise.
It's really really frustrating when the student's explanation that the evidence is "merely coincidence," and then later that the coincidence could be explained by having studied the book in high school.
I meet with the student tomorrow to provide an opportunity to rethink the denial. I'll be ready. I wish I didn't have to be.