My DH has had quite a few nasty criticisms from my parents. His mother, on the other hand, is a much nicer person, but is socially quite clueless. The most egregious criticism she's made of me (and DH) was very uncomfortable.
She's a very intelligent gynecologist who is very efficient and good at her job, but in her normal life she has absolutely no social skills or awareness. She often means well, but comes across as tactless or rude. After DH and I been going out for a year or two, she apparently heard (incorrectly, it turned out) somewhere that people nowadays always use Unreliable Contraceptive Method (UCM), and so she assumed that I must therefore prefer to use UCM also. So, always efficiently to the point, she brought this up the next time she saw me: over dinner. When we had intercourse, she began, we were wrong to use UCM because of the many medical reasons which she explained very factually and in great detail. DH and I were so gobsmacked that we sat in stunned silence for a very unpleasant minute while she went through the biological mechanics of the whole thing before one of us managed to speak up.
I know she did this because she was concerned for us and didn't realise it was inappropriate (seriously, she has no clue), but having my mother in law question me about intercourse and various aspects in tremendous detail was the most uncomfortable I've ever been.