I can't remember ever having a courtesy car, but the worst hire car we ever had was in Corfu, It was a Dhaihatsu open top jeep thing, and after picking it up, the first bump in the road saw the rear silencer fall off and bounce down the road in a shower of 'sparks! It was totally rotten. On the short trip back to the Hire Shop with the rusted rear box in the back, I noticed that the oil light kept coming on as we turned each corner.
I took the back box into the shop and said I think it needs a new exhaust, and 'oh by the way, the engine might last longer if you put some oil in it.
'How do you know it needs oil?' I was asked.
So I told them and they followed me outside where I watched their 'mechanic' eventually find and extract the dipstick. No oil showing at all on it.
Having said that, after being filled with 3 litres of oil, we ran around in it for a fun week!