I thought this would be a interesting topic. Here is a couple of pics on a car shell that I am re-building.
LH side A panel. OK rusted like most, but the "repairer"

just cut out the rust in both inner and outer, welded in

one bit of steel to both panels and then bogged the whole thing up, about 1/4" thick.
the mud guard was a ripper. It had a crease along it [side swiped something] so just fill the crease with bog. It took longer to remove the bog than to panel beat the crease. just a couple more little bumps to tap out and it will be bog free.
I have found some really good repairs on some cars, like the steel brake line that had rusted thru, so join it with some hydraulic hose and a couple of clamps.
same car had worn brake shoes, so easy fix, just grind off the part of the steel shoe that scraps the drum.
wheel bearings, angular contact type. fitted is the same direction and not back to back. I wonder if the driver wondered why his front wheel wobbled around with a grinding noise.
there must be plenty more stuff ups you blokes have found
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