This isn't the most idiotic thing I've done, but I think it accurately displays my lack of common sense.
During Year 7, in Food Tech, we were making apple crumble. Being me, I was peeling my apples with the blade moving towards my fingers. I ended up slicing my thumb, which severely impeded me for the rest of the lesson. Later, when it was time to grate the peeled apples, I grated the apples using the extra-fine grater, the end result being a continuous stream of apple juice going into my bowl. I honestly could not work out what I was doing wrong, until the teachers helped me.
At lunchtime, I resolved to eat my terribly prepared apple crumble anyway, just so all my effort would not be wasted. I did, but in the next class, I ended up with great stomach pains, and so excused myself to the toilets. Upon leaving the toilets, I realised I'd left my lanyard (with keys and ID card) in the cubicle. For some reason, I thought other kids would steal it, and raced back to the toilets, but could not find my lanyard in any of the open cubicles. There was one locked cubicle, so I asked the person if they'd seen my lanyard. A girl responded that she had not, but I took no notice of the fact that I was talking to a girl, and paced back and forth, trying to work out where my lanyard was.
Only after a minute or so did I realise I was in the girls' bathroom, and so tried to quickly leave, but alas, just as I got to the door, three girls on the other side opened it, and screamed extremely loudly.