Pickup mileage can certainly be wrong. It wasn't a big problem because I had unlimited mileage, I once got a car with 15,000 miles, but the contract was missing a digit and it said 1,500 miles. The return agent did a double take and realized there was probably a mistake before even mentioning it to me and said the previous return agent had problem typo'ed the mileage. She was also able to look at the history for the car and see that it had way more than 1500 miles when I got it.

Unfortunately, if they supposedly check the rental history for the car and find the starting mileage on your contract was consistent with the car's history, you are probably going to have a hard time fighting it.