It's happened to me twice: once the agency offered my choice of waiting or a cab to my hotel and, supposedly, a car from a nearby HLE the next morning. I was the only President's Circle member in the crowd, so I opted to wait...making it clear that I'd take the next car up of any kind/size. It arrived 15 minutes later (a Taurus or something), they knocked a few bucks off my rental, and off I went.

The other time, it was Alamo and they weren't going to do anything. I had a midsize booked and the lot was empty with people everywhere barking at the agents and milling about the lot. They brought up an SUV and parked it in the SUV aisle. Some guy with an SUV booking walked over there and started arguing with the agent saying he booked a ski rack, could they install a ski rack, etc. Amid all of the arguing about ski racks, I could tell the guy was going to stand his ground on that point...so before anyone could change their minds I jumped in and drove to the exit gate. The exit guard just processed it as a midsize rental and sent me on my way.

There have been a few other times where it's *looked* like it might be an issue...empty lot, people waiting, etc...but cars were coming up fast and it ended up not being a long wait.

I've had other problems, such as a Hertz location (Seattle) that was perpetually not prepping Gold rentals on time due to their own incompetence, not a shortage of cars. That's the only place I've habitually double-booked (Emerald Aisle was a 2-minute walk ahead from Hertz).

I honestly wouldn't object to something like a $10 no-show fee, as long as the agencies had good mobile apps to make it easy to inform them of a legitimate delay.