PO's are an amazing bread alright. In my 750 someone had "rebuilt" the top end but the engine had never been fired up luckily. Some 6mm threads in the head were helicoiled but the ones that weren't should have been as well, there were all sorts of different length bolts in the cam area, some Allen heads and one that was an imperial thread because it went in tighter. The tacho drive bolt thread in the head cover was stripped out and the only thing holding the bolt in was the locking tab, I've seen what happens when that bolt falls into the cam.
Recently I've been working on a Kawa900, I'm told that a qualified auto electrician "worked" on the wiring, the usual mongrel connectors were apparent, electrical tape is everywhere, under the tape the real debacle is hiding, there must have been a meltdown because the power wire that goes to the ignition switch had 4" of insulation that was just gone and taped up, also under the tape were wires that had been twisted together, some were soldered but some weren't including the previously mentioned power wire.
The current owner said the bike would cut out momentarily when on left lock, this was done by some tool who was a trained and qualified tradesman, so I'm never surprised when I see what PO's do.
And to be fair, let's not leave out a lot of current owners as well.