(1) When I bought the ute, the tray had more rust than solid metal due to the previous owner storing grass clippings in the tray for goodness knows how long. My solution was to grind out the whole tray and replace it with a sheet of 1.9 mm steel checker plate. Spent ages cutting the sheet to fit nice and snug and had a mate MIG weld around the edges to seal it all up. Did a great job too. About 15 months later I noticed the cross beam above tha axle hanging down at a weird angle. Seems that from the constant suspension pounding the shocker had caused the cross beam to weaken and break. Seems the factory had spot welded the beam to the tray floor for a very good reason. After welding the beam together again with some angle iron for strength and then welding it to the tray floor, problem solved.

(2)I'm not very mechanically minded and had to replace the diff centre as three of the crown wheel teeth had sheared off. Did the job perfectly all by myself and felt pretty good about it too. Went to show off to my mechanic mate and on the way home felt the back of the ute swaying around real badly. Checked the back wheels and found that I had forgotten to tighten one set of wheel nuts. Only one nut left on the wheel and that was only held on by one turn on the thread. Couldn't find the other three nuts and had to call the wife to rescue me as the wheel brace was back in the garage.

There are probably several other stupid things I've done over the years but I think I've read about most of them in this thread.