Underage fees are one thing, but policies like extremely large deposits if you won't pay for their insurance, paying surpluses for diesel models if petrol is unavailable (and they never are), having to pay for the first tank of fuel as well as the one you return it with, etc.

This, I think, is the biggest problem. You get told your rental costs 100 Euros on a website, you do that and turn up and suddenly you're looking at paying two or three times that on top of a deposit which must be paid by a credit card and may well be above your limit.

I think this is largely a problem with holiday destinations trying to fleece unwary tourists, I've had good experiences with local van hire companies who make things straightforward.