I've worked on, around, and sold parts for cars since 1995. I'm by no means a certified master mechanic, but I can say this much: I've never pretended to do a service that my employers charged a customer for. And I've worked for some shady places. I always strive to be honest, no matter the consequences. I've screwed up a few times, costing my employers money. I once backed a customer's brand new Bonneville GXP into one of the yellow-painted concrete posts on either side of the garage doors, crinkling the quarterpanel and putting some pretty yellow stripes down the side (the stripes wiped right off with lacquer thinner, but the dent didn't). My boss tried to make some lame story, but I interrupted him and told the customer that I was talking to a coworker and just plain wasn't paying attention. She thanked me for my honesty, the boss paid for the repairs, and I got one hell of an *ss-chewing.
Anyway, I've heard and seen some horror stories too. The most recent happened to my parents. My mom worked for a business that was right around the corner from a towing/repair shop. Her parking spot was literally right across the street from the shop, so my parents didn't have to make a special trip to take a vehicle to them for any service, my mom would just drive the vehicle to work and leave the keys with them. They never once tried to screw my parents over, and they were on a first-name-basis with every employee there. The company eventually decided to do towing only, and sold the building to some guy. My mom's Sebring's engine started making an odd noise, so my parents decided to give the new owner a try because of the convenience. He was the owner/sole mechanic, which is a big red flag, in my opinion. He took her car in, and took a look. After 2 days, he said he had no clue what the noise was, that his best guess was a timing belt tensioner or guide, and given the mileage on the car, it would be a good idea to change them and the timing belt anyway. My parents agreed, and he started the work. After a week, my parents called to see what was going on, and he said he was waiting on back-ordered parts, and would call when they came in. Two more weeks go by, with him not answering the phone, and they finally talk to him. He got the parts, but got hurt working on another car, and wouldn't be back to work for another 2 weeks. Two weeks later, just as he said, he calls. The car was actually done. My dad picks it up, and as he's leaving, the guy tells my dad that there was a coolant leak that should be checked out. They look at it together, and the radiator was leaking, but it didn't look like it was intentionally damaged to my dad. So he told the guy to go ahead and fix it. The next day, it's done. Again, my dad picked the car up. This time, as my dad is getting ready to leave, the guy tells my dad that something is wrong with the brakes. They felt spongy and the pedal went almost to the floor. Now my dad is very very suspicious, because the brakes were practically new and they were 100% perfect when the car first went in to this shop. He tells the guy that he'll drive it home to see how it is then let him know. They were exactly as the guy said, spongy with a very low pedal. Dad took it to my uncle's the next day and they took a look at it themselves. The first thing they noticed is that the dust caps on the rear brake bleeders were gone, and the tips of the bleeders were nice and shiny and new-looking, as if the caps had been removed very recently. The bleeders weren't very tight, and there were signs of brake fluid trails coming from the bleeders that had been wiped up. So they bled the lines, ultimately putting almost an entire quart bottle of brake fluid into the system, and miraculously, the brakes were fine. Wanting to be 100% sure there were no other problems they might have missed, dad took it to another shop in town. This shop has been around for decades, and is more than reputable. They thoroughly inspected the car and found nothing but bad upper rear strut mounts (Chrysler made them out of aluminum and they corrode and break apart, a common problem with those cars). The service manager asked who the sketchy mechanic was, and when my parents told him, he knew all about the guy and said that he had "heard a few things about him", but was professional enough to not go into detail. M parents planned on confronting the guy, but decided it just wasn't worth the hassle for them. But a month or so later, my mom couldn't resist going to the guy and asking him if he had any brake bleeder dust caps lying around because hers were missing.