When should you not report plagiarism? It seems that most of the ethics and plagiarism questions follow the same basic archetype:

Q: I think there's some plagiarism going on, and I don't really want to report it, but I think I should. Should I?
A: It'll be tricky, but yes, of course you should report it. Consult your institution for more details.

My question: is there any case of academic dishonesty (including plagiarism) that a student/TA should almost certainly not report?
When you're not sure is probably one situation. You're accusing someone of dishonesty so you'd better be able to prove it.