Was putting on a new intake manifold and solid valley tray on my car and the paper towel that I had blocking off the number 2 intake port came out. Said screw it and promptly dropped a 3/8" washer on the pan that bounced right into the port and sure enough the valve was open. Heard it plunk in the bottom of the cylinder. A Friday night job turned into a weekend job.

The first motor I built for a customer was a early (pre 1962) 383 motor in a 1963 dart that he had installed and made the motor run before I met him. It had the early cast iron 3 speed tranny with the ball and trunnion rear U joint, I sold him a later1964 model 727 and crankshaft out of a 1964 361 motor and was changing the crankshaft to make the later tranny work in his car, he wanted to watch me and I let him, he smoked and had drop ashes into the motor several times: rant:
It was in Hesperia, CA in the winter when I was working on it at night after work in either late November or maybe early December in 1971, it was cold, in the low 40S F and he wanted to go inside my home and have a cup of coffee to warm up, I was done torquing with the first six rods when he started whining about going inside. When we came back out, he said we were done so he was going home, I screwed up and didn't retorque any of the rods after he left, let along finish torquing #7 and 8 rods
We started the motor up and it ran about four minutes before one of the rod nuts unwound and allow that rod to separate from the crank and hit the block cracking it
That was the last time I have allow anyone to watch me work on motors, tranny, rear ends or any other part of a car project other than at the tracks
That is the worst thing I can remember screwing up on car projects, motors and so on at this time

Almost the same. I was checking for loose connections on an MSD 404 in a Hemi Super Stocker when the helper thought he should help and turned on the ignition switch. I straightened right up hitting my head hard on the hood. I fell to the floor twisting my knee, skinning my elbow, and hitting my head again on the concrete. The helpful assistant jumped out and ran around thinking he might have killed me. I was ok but I couldn't stand up for a while and my arm shook for at least 15 minutes. Capacitors can be very dangerous. I remember reading that those race Msd boxes could through a spark 7 inches through the air. I'm a believer.

Hmmm...I tried to find the weak link in a 9" w/mini-spool, a good starting line, and fresh slicks. Turns out it's the mini-spool. Not a total loss. Between the slicks and the rear u-joint, I saved the housing, the brake rotors, and the u-joint yoke.

An idiot who used to work for me had a boat with BB Chevy. He only new to how pit gas in it
We normally do not get cold freezers but were. Someone told him to make sure it has antifreeze.
Next warn spell went and tried to use the boat. He had poured it down the vavle cover so it blew it up.

probably my MOST dumbest of the dumbest moves involves looking at something under the hood of my old man's car.
i wear glasses, and of course, i can't see with 'em, and i can't see without 'em..............
after only a couple of minutes, all was well with his car.
shutting the hood, i start looking around for my specs. look and look, can't find 'em..........
finally, opening the hood, there they are.........mashed flat on top of the intake plenum.
of course the old man busts out laughing !
it took me almost two hours to straighten them out enough to wear again. not bad for a couple minute under hood check out..................

I had just finished putting together a Gen 2 Hemi, first engine I'd built from a bare block by myself, and more money than I'd ever put into anything in my life. Everything had to be right the first time, as I couldn't afford to screw anything up. I got it in the car, put a little fuel in the carbs' throats, and tried to fire it up. It started... but was only running on two cylinders, like a shaking like a friggin Harley Davidson. Horrified, I shut it off, check things over, verify the distributor is in correct, and try again. Same result, over and over, until I foul out the plugs. Off I go to the parts store for a new set. Same results. Frustrated and discouraged, I quit for the day. The next day I have my brother look at it with a fresh set of eyes. He quickly points out that the spark plug wires are on the cap in the correct order... but reversed, so only two cylinders were firing in correct time.