As a teenager lots of us hopped up our Mustangs and Camaros. There was a guy in our shop class who was a little behind, not the sharpest knife in the drawer. Borrowed some tools to work on his car, said he was putting on new headers, hi-rise manifold and a honking big carb. Had his manual (with pictures!), it would be an afternoon job, we'd all see/hear in the school parking lot "tomorrow". Called a couple of days later complaining my torque wrench was faulty, it had snapped 3 studs on the manifold. While bolting the carb on. After a few questions like "why the f&*k would you torque a carb on" and "after the first one broke, why did you continue and break 2 more?" he insisted the manual had given all the correct ratings.
It took over an hour to explain to him the difference between "lbs/in" and "lbs/ft".
A few weeks later he called for help, confused why his engine wouldn't start and made funny sounds while cranking after installing a performance camshaft. It's a pretty easy job on a Chevy, but it was (not) a surprise that he had indexed it 180 degrees off, bending every pushrod.
Same guy a couple of years later had a minor accident after bleeding his brakes. Yup, stripped the left rear bleeder by cross-threading it. "Well, it was in tight and wasn't leaking when I finished". Luckily, he wasn't on the road, and only hit his dad's car in his own driveway after going around the block to be sure everything was working.... which, of course, wasn't. The bleeder had ejected after a couple of stops leaving a nice trail of brake fluid.
In the 1980s he worked for a few years as a parts and service writer at a local dealership. Seriously.