Anything I borrow, I try to return in the original condition unless the owner specifically states that it's ok if it gets damaged (and even then, I try my hardest). If I damage it, I notify them and compensate them within reason.
The only exceptions to this, IMO, are "freak coincidences." If I borrow someone's vehicle for one reason or another and the transmission blows out of the blue without me abusing it, I will provide all of the details and help where I can, but I won't assume responsibility. This is the #1 reason why I try not to borrow stuff - I don't want to deal with the aftermath if something happens that isn't my fault.