I rented a car while on vacation in Switzerland and yes I screwed up and put regular gas in a diesel car. It ran funny and I figured it out and stopped down the street. Called Hertz, they sent a tow truck which towed it to a shop. Took a taxi to the hotel and the car was ready the next morning.

When I picked it up, the lady at the garage said that Hertz insurance ("assurance") had taken care of everything. I asked her out of curiosity how much it cost and she said 300 swiss francs (around $300). So at least I didn't worry about having to buy a new engine for the rest of my trip.

When I returned the car at the end of my trip the agent said he couldn't give me a receipt yet. When I got home I was surprised to see they had charged me 977 swiss francs for the repairs. I immediately put in a dispute on my amex.

I called Hertz and was kind of given the run around. They told me the breakdown was:
600 chf "wrong refueling"
157 chf refuel car
150 chf penalty for wrong refueling
70 chf for the service call
(chf is swiss francs)

I called the garage and was also given the runaround, saying they couldn't find the invoice.

So my guess is that maybe the garage ripped off Hertz or something. I have no idea. I should just be happy it wasn't $3 grand to fix.

I am still waiting to see what amex says.