The biggest DIY fail I ever witness (and took part in partially) was letting my friend put my wheel back on. I went in the house to grab something to eat and told him to put my wheel back on. He used an impact gun to put the lug bolts back in (didn't even thread them in first) and stripped the hub out in the process.
We went for a drive, and about 15 minutes into the drive, we're going about 45 mph down the road, and all of a sudden my driver's side wheel falls off and goes shooting down the road in front. We slid on the brake rotor (which was shooting up sparks like crazy) as I slammed on the brakes and we came to a stop on the side of the road.
Some guy drove by us and yelled out his window "Should have bought a Honda bro!" (which made me laugh out loud).
Anyways, the moral of the story is never let your friends do something for you, unless you're watching. I'm lucky we weren't on the highway or a busy road, otherwise it could have been serious. There were a few vehicles that witnessed it, and man... I would love to have been the guy to witness this from another vehicle. It had to have been funny as hell to see.
This wasn't on my Jeep BTW, this was about 16 years ago on my first car, a 1993 VW Corrado.