I had a plumber crack the porcelain on the sink when tightening the drain. They told me about it and bought me a new sink. That's my plumber until I move. In another town the same thing happened and they put some silicone on it and didn't tell me. That plumber was never called again except to tell them why.
The one near my house pointed out that there was chunk of EGR hose missing and the rest were ratty. They could replace them all for $280 (above the mininum required to address a failed test).
It was the hoses. Went from above the limit on two tests to 10% of the limit. I used that mechanic until I moved across town.
I worked as a bicycle mechanic for a bit (lots of bolt cutting, more cross threading than I wanted), but also spent a cold winter working with my dad on a vintage car and I discovered that I don't have as much nostalgia for car work as I thought I might. For the same reason my boss pays for licenses (because you can yell at them and make them pay for the repairs) I pay others to do some of my handiwork.
Despite still being able to turn a bolt the right way while upside down and with the bolt facing away from me, I figure I'm almost as likely to get a rude surprise due to my own stupidity as theirs. If, and this is a big If, I've developed a relationship with the vendor, instead of just driving down to the Jiffy Lube where I'm nobody.