See I have the exact opposite experiences as you guys. You deal with stupid customers, I deal with stupid mechanics. I'll just go down the list:
-when I got my car, the small dealership replaced the tranny and put too thin of a gear oil in. Blackstone tested and everything, now half my gears grind constantly
-timing belt job, months ago. During my inspection a few weeks ago they found a bolt finger loose on the timing cover.
-had my valve cover gaskets replaced because of internal leaking. Now they both leak externally, but "it's not the valve covers" -_-
-had my rear wheel bearings replaced by a local shop. I get to work the next day with smoke pouring out of my rear drums, the shoes were adjusted too far out. Also, they left my drivers side lateral link loose (I had to tighten), and 3 bolts on the rear differential loose, because they put in new axles too.
Thats not even all of them.
It's very frustrating. It's always awkward going back to tell them the mistakes. I honestly get paranoid when having people work on my car now. I try to do as much DIY as I can, but I just don't always have the right tools.