Problem not exactly caused by another service person, but with something that they sold
I had built a pretty good computer for my dad, and he had decided to buy a new printer. Now, had he asked me first, I would have told him one make not to get - hint, the name is an anagram of the name of a Barbarian - but he bought one from the friendly guy at our local computer shop. It just would not print when attached to the computer, and for some reason dad was not prepared to get the guy at the shop to deal in the first instance.
(Note, I'm not saying that a printer by the firm whose name sounds similar to a horse racing venue, or one that sounds like a sauce brand, would have worked in this case, but I am saying I'd have advised against what he went for anyway.)
I took a spare machine with me, knowing I might need to do a backup at least. In the end it turned out to be a rather good job I did.
After a backup, nuke and pave, the printer *still* would not print. I swapped the, fortunately similar, motherboards between my machine and dad's, and the printer worked. So, now dad involves the guy at the shop. Obtaining a new motherboard, CPU and two sticks of memory, I set about rebuilding based on that. All went well, until I installed a sound card. Now all manner of faults were occurring, lots of app crashes.
On this occasion I had to admit defeat, after several attempts. The guy at the shop, as he had supplied some of the parts, was very willing to help, but not exactly of the opinion it could be down to what he'd supplied, until he tried it. The motherboard performed perfectly, until you added a sound card - any sound card, not just the one I was trying.
The machine came back with yet another replacement motherboard, and with dad's sound card fitted and working, and the printer also working. The rest of the weekend was taken up with restoring old files and reinstalling software. This turned out to be the third in a sequence of "it must be your mistake" incidents with this supplier that had culminated in an apology. After a chat with a friend who had helped set him up in the first place, he even offered me a few "near end of line" parts free.