I reserved a car in Cancun for a week (two months in advance) through Kayak, for $150. When I landed in Cancun, the Advantage desk had no record of the reservation.
(The large group of people in line behind us had the same problem, and they had reserved two cars.) As a result, I had to rent through Avis for $550.
I just called Advantage to complain, and the woman gave me a website to submit a claim, saying Advantage would give me a credit for the difference; however, the site (global.advantage.com) comes up as untrusted by Firefox. Grrr.....
Monetary Loss: $400.