I've had this 2x in the last 2 weeks and wondered how (in 2024) this is still a problem. Two recent examples:
1. Had a car for `2 days. Upon return, they added an extra digit in the return mileage, so it was basically 10,xxx miles driven over 2 days. >200MPH for the entirety of the rental.
2. Picked up Car X from an airport for a 1-way rental (I had it assigned at the Preferred counter, and had it scanned on the way out of the lot when they checked my license). Car had ~22,000 miles at pickup. About 2 hours later, I got an email/text that I was assigned to Car Y and it's ready in whatever space. This one had ~900 miles on it. CHM desk said I have to fix it at the pickup location. I was already >100 miles away from the pickup location and I had a printed contract, so I didn't bother trying to go back. When I returned the car the next day to another location, the agent was baffled at how to fix it, so she just checked in "Car Y" (the one I wasn't in possession of) and used the actual mileage from it. So it showed I drove >700MPH for the entire rental.

My corporate rate includes unlimited miles on most rentals so I didn't really care, but crazy to see there's not a flag to check when that happens.