It's a fine line between optimising the yield by having as many of your cars out earning rental income and not actually running out and adversely affecting your customer service, and as others have said, I guess it is inevitable that there will be occasions when it all goes wrong and the situation the OP articulated does occur.

I have to say that it is not helped when you get people who make reservations, and then in a fit of pique cancel them in real time and walk across to another provider. There are plenty of people on here who admit to doing this. Rental car companies will no doubt plan for a certain percentage of reservations to go away, so when they don't, I guess they find themselves out of cars, just like when the airlines overbook and they run out of seats.