I worked for about six months as a contract manager, looking after the cleaning contract (back in the day it was ASIG, before OCS and the current provider). Part of my job involved inspecting aircrafts, before and after cleaning. It was only six months before I moved on, but boy wasn't it an eye opener.
I've seen blankets used as cleaning towels for toddlers, emergency exit areas soaked in urine, hand basins used as latrines (very popular on Indian routes), petty vandalism (people perforating the IFE touchscreens with ballpoint pens, punching holes through window blinds, graffiti on the First class seat shells), chewing gums sticked into the seats, especially on the leather armrests of the new First, duvets flushed down the toilet, barf bag emptied into the seat pockets, nappies stuffed in the toilets or under seats... My absolute worse was a string of cases when an insulin syringe had been left, without the cap, in a seat pocket, pointing up. It happened 3 times within months, always in Traveller Plus, always on the same route. We tried to identify the scumbag that was doing it, because it was clear that it was being done on purpose, but unfortunately we didn't catch him, or her. Two poor cleaning ladies had to be taken to hospital to be checked for HIV and other problems.
It really was an eye opener on how disgusting people can be.