I think mine take's the cake for stupidity award..
Ok this was about a year ago, when I first started to get into supra's and didn't know my head from my ass when it came to working on car's.
I was trying to set the timing, so I got the socket set out and turned the crank to the tdc marks, made sure everything lined up, hooked up my timing light, got a phone call from a friend and walked away from the car for about 5 min, came back and jumped in and started it without checking things..I hear a horrible "CLANG, BANG, CLANG, CLANG", I quickly shut the car off and get out and on the ground there is my crank pulley bolt.. along with my wrench...I had forgotten the wrench on the pulley bolt...luckily all it did was knock the line to my oil cooler off..
...I still feel like a moron for that one..
I've also forgot the afm plug...get in try and start it, and start panicking..LOL