All of us do mistakes. Some of them are even pretty funny. I think we could put here some mistakes we or our friends have done but not recommend to anyone else. Tell what you tried to do and what was a payoff. Start every story with words 'You should never..."
You should never try to bake raw, frozen croissants in a microwave if them are aimed to bake in a oven. Payoff: Black, smoky croissant which was appressed in a plate under it. The plate cracked too.
You should never keep your sugar and salt in containers which remember each others. If you do that, you can experiece an awkward surprise in a some morning when you're tired and trying to sweeten your morning coffee...
You should never try to stand up fast if you've worn jeans with wide legs. Try to do that and it's sure you'll topple those legs. If you're sitting on the chair, bed etc. you also end up topsy-turvy on the floor.