Buddy of mine used to work at a quick lube shop. Lady brought in her car complaining about poor engine performance. Said he opened up the oil drain and it came out like tar. He asked her when and where she got her last oil change done. Her response? This was her first oil change. Car had something like 35,000km on it.
Took him the whole day just to drain the thing. Kept the oil in a jar on the front desk to show people why they need to change it.
I used to work at a car rental agency, and had more than my share of facepalm moments. One time this lady phoned in complaining about a red light on her dashboard. I asked her what it looked like. "Oh, it's a big red exclamation point in a circle." She had been driving around all day with the parking brake on.
Car interiors also used to come back with all sorts of head scratching messes. Filled with food, garbage, mystery stains, cigarette burns. Pet hair was always the worst. It gets caught in the fabric so it would take about half an hour to vacuum it all out. About the time it took me to clean two cars.
The worst mess we had to deal with was the catering company who left a plate of roasted garlic in the back of one of our cargo vans. We were closed Sundays, so it sat festering in the hot sun until we opened it up Monday morning. Never did get the smell out.