Oh yes... I know whereof you speak!
My husband and I used to live in an apartment complex before we bought our house. About half the parking spots were numbered, and there was 1 numbered spot per apartment. We had 2 cars, so whichever of us got home first would leave the numbered spot for the other one. No big.
So I got home early one day and some random car was in my numbered spot!
It was somebody's boyfriend. I just parked in an unnumbered spot and left a note on the car that numbered spots were reserved for residents and next time we'd have him towed. I never saw his car in a numbered spot again.
What always bothered me, is that people would park in clearly marked no-parking zones, like in front of dumpsters, in front of the mail kiosks, etc. The apartment complex staff DID NOT enforce the parking rules. People could enforce no parking in their own spots by calling the towing company, but as for the rest of the complex, anything went.