A few years ago, I had service done at a local Mercedes dealer, which included (according to the service advisor), replacing the air filter. I had no arguement as it was about 35,000mi old.
A few months later,, at the time of an oil change, I asked the mechanic to check the air cleaner as part of the routine. He opened-up the filter housing and surprise, no air filter. I was there so I saw with my own eyes.
I went back to the dealer to explain what I saw. Their answer: Impossible, not us, can't be, no way, can't happen, it's a joke, you should have check it before you left, it's your fault, somebody stole it, our warranty is for 30 days on routine service, we always double check our work, the technican who worked on your car has 15 years experience. and bla bla bla.