Mine isn't as stupid as a lot of others, but I thought it was pretty funny...
Adding some security features to my old car, I thought I'd take it out to our land so I didn't disturb anyone while testing the alarm system. I was adding in something that would roll all the windows up when somebody got close enough to my car, then the full alarm goes off when you stick something inside the car.
I'm walking around the car, while it's armed, trying to calibrate the sensitivity. I put my whole upper body into the driver window, forgetting that it's armed, I set down the remote on the backseat floor, and pick up the motion detector to tweak the settings when the windows start going up. I get out but grab the keys from the seat and my left hand ends up getting stuck in the window.
After a few min, I use the key to open the door, which sets off the full alarm and put the key in the ignition to try to roll the window down. Since I wasn't done wiring it all up, the signal to roll the windows up stayed on and I couldn't roll the window down. So I had to put the key in, hit the valet button and disarm it, all while in an ackward posistion and freed myself. I was more cautious from then on.
Another time, shortly after moving, I came back from an hour drive and deceided to change the oil quick before I had to goto the airport. I had a recyclable container, with pan that drains into the inside. So I quick jack the car up and pull the oil plug and it drains. Then I notice the oil isn't draining into the container, and in a little bit it's going to spill over onto my new garage floor! With no tools, except a socket wrench, I had to put my hand into extremely hot oil to undo the plug on the container. I still dripped some oil on the floor and burned my hand slightly, but no major oil spill!