What stupid things have you done?

Years ago I was just starting out as an apprentice tech at a local ford truck dealer. first time changing a fan clutch in a 7.3 diesel. The shop foreman handed me the labor tag and the fan clutch wrenches and asked me how I was going to go about changing out the radiator and installing the new fan clutch. After I explained my whole procedure in order he told me one thing to make sure of is that I tighten the $h!+ out of the clutches hub nut with a hammer or something to really torque it. I said ok and was on my way. I got the whole job done and me thinking I was a young tough guy surely strong enough hand tight would be perfectly fine. I fired the truck up, checked for leaks and called the foreman over. He took a look and told me it all looked good and to just shut the truck off and start putting all my tools away as it was about the end of the day. I go to shut the truck off and sure enough when I did I hear a banging sound then see something fly up spinning like crazy, smack the underside of the hood then shoot back into the engine bay! Oh F#%$! It took out the shroud, radiator hoses, radiator, cooler lines, and fan. Foreman hands me a hammer and says " well....... I bet you don't make that mistake again."