This doesn't work if the failures are correlated and very often happen to a certain group of people. Imagine if the system was flawed and assigned all tickets

This doesn't work if the failures are correlated and very often happen to a certain group of people. Imagine if the system was flawed and assigned all tickets with an unrecognized license plate to NULL or 000000. THe owner of that specific plate would have to spend their entire life contesting tickets. A less extreme case of this would be a badly-trained model that always detects certain kinds of cars as occupying bus lanes, even if they're parked legally. If you're in the tiny minority that has one of those, you can't park anywhere without getting a ticket which you must contest. If those cars end up statistically more likely to be owned by the poor / black / homeless, now the local government has a scandal on their hands.