Originally Posted by
swag
With no deposit and no penalty for no-shows, it's hard for the car companies to manage their fleets.
For this same reason, why wouldn't you have a "backup" reservation with another company?
Another option: Start a thread here to see how common it is for a specific rental company location at a specific airport to have this problem. It'll be a small data set, but it might help you avoid the sites that are most likely to experience this problem.