I'll start...
Our rather overzealous PTA informally announced that if children didn't bring in a raffle prize donation, they'd have to wear school uniform on non-uniform day.
One mum decided to send her kids to school in full uniform in protest, announcing it to the WhatsApp group. The group was silent for a change, with people not knowing quite what to say.
I could see what she was trying to do, but it seemed a bit harsh to inflict it on her own kids, especially as everyone knew the school wasn't actually going to enforce the rule.
Do your WhatsApp groups have any stories?