My largest "oops" centered around my first attempt at having a diesel car. I had picked up on Ebay, a rather grungy old VW Rabbit Turbodiesel. I drove clear up to Minnesota to pick the car up. Once I got it home, I was tinkering under the hood to find what I thougt was a sticking injector (running rough and a mosquito fogger). While listening to the injectors through my handy dandy 2 foot 3/8" drive extension, I reached down and cracked the throttle open. I was immediately greeted with the sound of valves and pistons getting to know each other.
I then commenced to tear down the engine to get the head fixed, and do a rering and bearing kit while it was torn down. After a $400 head rework, I put the car back to gether. No fire. After exhausting all possibilities, I had to take it to a VW shop. Compresson was down to 150 pounds across the board. It was going to require new pistons to get rid of the nice new bowls that were made in the tops. At $1600 for a new set of TD pistons, The car got donated to the local high school instead of repaired.