One thing I learned really quickly renting in Europe is to take pictures of the rental before driving off. Last time I rented a car there was a huge scratch on the side that wasn’t included in their pickup form. When I returned the car they were ready to bill me for it until I showed the picture and their response was: “OK, no worries then, all is good”. From the reviews I’ve been reading people often get charged this way and I’m wondering how many people end up paying for the same damage.
I just had this in Hawaii (Big Island). Dollar car rental dude checks my car when I return it and decided some scratch on the fender was new (I'm pretty sure it was not). I had full insurance and I told him he can figure it out with the insurance company, that I was in rush to catch my flight, and he can do whatever he wants. They didn't do anything. They're just looking for some sucker they can charge extra on some random thing.
I've had a lot of rental car experiences. Usually it's fine. Taking photos sounds like a good idea.
