Ooh, i've got one... but this may not
really qualify as a PO bungle.
I bought a 1971 CB 750 from the original owner in Sept. It was still wearing a windshield/fairing

that was installed by the dealer in midtown manhattan.
It hadn't been ridden since 1981, or it definitely had not been registered.
Cleaning up, and doing my basic autopsy, it was fairly easy to get running, and so complete that it was difficult to figure out why it was parked... till i took it around the block. It had a vicious wobble and would track with even the smallest groove in the road.. at any speed.
I pulled off the fairing and discovered that the accessories guy had mounted the fairing to one of the tree-top fork tubes and cranked down on that bolt, cracking off the clamp on the LH side... I bet that bike NEVER rode well. I can picture a trip or two to the dealer followed by some head scratching, as you'd never see the crack with the windshield on.