I have been stung before at the local MB dealership - never again. I have also witnessed this. Just this weekend I put a block heater in my car (NEVER AGAIN, thank you!) and the motor mounts that supposedly were replaced two years ago were never replaced - I suspected as much after they "replaced" the plugs and wires when I bought the car. They got me because I just purchased the car there and 2 weeks after it was missing horribly. I took it in for them to work on, thinking it was something horrible like a blown engine, and was relieved to find out it was just the previous items which were not covered by warranty. They agreed to do this 50/50 because they had just "serviced" the vehicle. Keep in mind this was when I was "mechanically illiterate". They cleaned the spark plugs, did not replace the wires, and, evidently, did not change the motor mounts.
I also have seen them "take" a well known dr.'s wife who owns a '97 s600. She came in for a routine service. I was there for warranty work or would not have been - you can be sure of that. She drops the car off, leaves for lunch, and comes back two hours later. Her car was never moved but she was charged big $$$ for the work. I actually stopped her and told her to examine the way her car was parked and the seat position. She realized that it had not been moved and went to the service mgr. He gave her the lame excuse that it was an oversite and that he would do it immediately. I told her she should insist on watching at least part of the work being done on the car.