I rented from a local branch of Enterprise in Phoenix earlier this year and they were out of cars. There was nothing I could do except sit around for an hour waiting on them to shuttle in more cars from the airport.

Thankfully that's been my lone such experience this year. I will note that having seen huge lines at various leisure airports I'm only renting from Hertz and National where I have top tier status and don't have to wait at the counter to hopefully bypass some of these issues.