I've recently set up a business working on other peoples vehicles and it does seem to happen far too often, garages charge for work but then don't actually do anything.
Just off the top of my head there was the Chap with a 2.2 petrol Signum who's low mileage engine had started to make a ticking noise, he needed a new timing chain kit and upon strip down it became apparent that the oil filter had been in a LONG time, yet he'd supposedly had the car serviced at his friends garage just three months before...
Also some Garages will do work without actually fixing the fault, another Bloke had rough running and e.m.l. on, on a Punto, one of the spark plugs was coated with thick black sludge so I advised probable piston rings and cleaned up the plugs for him which solved the rough running so he was happy... a few weeks later he rung me to say his car had failed the M.O.T. on the emissions and the garage had said it was because his cat was not working efficiently. He asked me to accompany him to the garage whilst he dropped of his car to have the work done. I explained to the Mechanic about my fears that if they replaced the cat and lambda sensor then they would soon become contaminated due to all the oil the engine was burning, he said they would check the engine before fitting the new parts!
The bloke ended up paying around £200 for a cheap cat and lambda sensor. His car still has the e.m.l. on and is still contaminating no.4 plug! I don't know how they managed to get it through the emissions test.