The stupidest thing I have ever done...
Jumped the terminals to start a car I was planning to rebuild once, not realizing that it was in drive, it immediately drove itself into the wall of my house, directly under my bedroom window. Luckily the front bumper was already off, and it hit the trash can first which absorbed some of the impact, I looked up at the window to see the scowling face of my first wife, and decided then and there that I would just admit I screwed up.