I know people whose injuries are pretty much completely invisible. While they may qualify for a parking pass, because as a result of their injury, they might forget where they’d parked, but if they’re alone, there’s no way that they’d use it. If someone were to see them get out of their car, alone, when they’re parked in a disabled spot, chances are they’d get called names, and yelled at. There’s nowhere, at any time, that anyone would say anything to me. I drop something, and within a few seconds, someone will offer to get it. I was in Wal-Mart recently, scooted to a long line, and the person in front asked me if I’d like to go ahead of them. Granted if I did, it would be bad of me to take unfair advantage of them, but they offered.
If someone does something that you think wrong, or something like using a disability-spot, don’t get mad. Ask them why they’re there. If they have a reason, then say that you “see” their invisible disability. If they don’t, then ask them to move.