I was old enough to know better to check my pockets, but I once let my iPod go through the washer and dryer as a teenager and ruined it. Check your pockets all the time before washing!
I also can't count the times I myself had the trapping/skinning gambrel hanging up and forgot to slide it over, and ran face first into it not paying attention where I was walking. Watch where you're going!
Once left a bag of chicken feed out and not in a closed container, only to come into the building and see an absolutely GIANT roof rat scrambling to pull itself out from inside the bag. Good thing there wasn't much left in the bag... Keep your feed contained from pests!
One morning I got up and somehow managed to do everything backwards about making coffee as I was half asleep. I turned the pot on and stepped back, wondering why it smelled so weird, only to realize I never even put the water in. Pay attention!
My absolute biggest, stupidest thing was almost losing a finger because I was being irresponsible years ago. I had an absolutely massive bowie knife I used to carry to the river and use for multi-purpose things. Once I had it at a bonfire and was cutting up limbs to add to stoke the fire. I don't know what or why, all I know is that I cut towards myself and cut my left index finger down to the bone. To spare more details, let's just say I'm thankful to God I didn't lose my finger, and I've got a scar to remind myself and others about knife safety for the rest of my life. Cut away from yourself - pay attention - be responsible - be safe - use caution. There's so many lessons to be learned from this!