hmmm.... I can't remember specifically but it's a pretty safe bet that it was telling someone his opinion is idiotic without padding my sentence with all the unnecessary qualifiers people seem to think are important in such situations: "I think in my opinion that it seems your opinion is idiotic, with all due respect, etc."
I think this could elicit lots of answers so, same Q, or better, not necessarily the last, but a notable occasion where someone has been offended by something you've said (with extra points if it wasn't intended to offend them)?