> sometimes they don't have one when you arrive.

Or, if they are Hertz, they might have one but refuse to give it to you. This happened to my wife. In spite of payment already being made to Hertz corporate online, the local agent wouldn't give up a car for a one-way rental. Hertz corporate was less than useless, telling us their system said was a car available, and suggesting we pay them hundreds of dollars again and go pick it up. When I asked the woman from corporate whether she could actually guarantee we would be given a car, she said she couldn't. When I suggested she call the local agent, she said she had no way to call the local office. Unbelievable.

Since it was last minute, there were... as you said, no cars available at any of the other rental companies. So we had to drive 8 hours to pick her up. Then 8 hours back, which was the drive she was going to make in the rental car in the first place.

Hertz will hurts you.

I personally would have changed it to a round-trip then just returned the car to the other Hertz location and let them figure it out.

Hertz this time, but things like that have happened with every rental company I know of.