It depends, whose fault is it, or whose irregularity was it?

I wqould think the renter would have an after the fact legal case against a rental company that refused to honor a reservation when the renter adhered to all his conditions.

(This is not a complaint) I once showed up early for a rental. The company gave me the choice of waitingg until my appointed time (about 4 hours) or paying more for an immediate upgrade. That was the only time I saw a huge line of people waiting for cars there which line I barely beat. I chose to waait (they let me ride the shuttle back to the airport where I got dinner) and the car I reserved was ready on time. Other times at the same company and location they have graciously given me cars early with no extra charge. This was a "second tier" brand name, Payless.

If the renter arrives late, all bets are off. The company might have or choose to have nothing available or have only walk up rates available, of couse at the risk of gaining a bad reputation however fair or unfair that may be.

Travel tips:
http://members.aol.com/ajaynejr/travel.htm