Ones where posters diagnose other people's children. Particularly with autism.
OP: My 3 month old baby isn't talking yet, should I be worried?
Poster: Classic speech delay. Not saying he is autistic but you should probably get him tested.
OP: My 3 yr old put his shoes on the wrong feet.
Poster: Classic sensory seeking. Probably get tested for ASD.
OP: My teenager leaves the lights on when he leaves a room.
Poster: Sounds like he might have autism. Have you had him tested?
OP: Husband doesn't like big parties.
Poster: Your DH probably has high functioning undiagnosed ASD.
Also ones reacting to MILs and their grandchildren...
OP: It's our wedding anniversary and MIL has offered to babysit so we can go out for the evening.
Poster: DON'T DO IT OP! You should pay for childcare, your MIL just wants to "accidentally" call herself "mummy" when you aren't there and whisper in your sleeping baby's ears what a shit mum you are.