Describing things like colouring in or jigsaws as hobbies.
A terrible creeping social insecurity about whether your children are 'doing enough extracurriculars'.
A (probably related) delusion that private school produces magically confident, well-rounded children, and that if you can't afford it, you should post on here for tips on how to replicate the 'private school effect' via viola lessons, reading that weird doughnut-faced man's etiquette advice (Willian Hanson?), discussing current affairs over breakfast, and Guides.
Everyone should have a Quooker.
A complete inability to handle the most ordinary situations involving other people, whether that's a too-chatty colleague, the school run, or a wedding invitation you don't want to accept. These are all regarded as requiring the kind of negotiating skills used in hostage handoffs.
A widespread blindness to the role of confirmation bias, cold-reading and forgetfulness in encounters with 'psychics'.
And some funny, kind posters who give well-informed advice, or a brisk kick up the ass, as required. The people who remote-diagnosed my toddler and gave me tips to help him when we were on holiday in a remote place have my undying gratitude. And though I'm not an adopter, I've periodically read the Adoption board for years, originally looking for ways to support a friend who'd adopted her daughter, but have also found it an incredibly good source of wisdom about parenting the child you have in general.