This picture shows how some types of ANPR errors occur. This occurred at the CitiPark Gade car park in Watford.
The second letter 'L' has been misinterpreted as an 'E', possibly because of the faint smear of dirt on the plate. This is a barrier controlled car park, so this could cause a minor problem on exit if the misread does not happen a second time, and different lighting conditions cause the ANPR software to correctly recognise the 'L'.
In an ANPR only car ark where there are no barriers, this could cause the software to record a car which entered but never exited. Normally this would be to the benefit of the motorist, as if they overstayed the ANPR would not know, and no ticket would be issued.
Problems occur if the motorist visits twice, and only the first entrance and second exit are detect. In that case the keeper would be issued a ticket for an overstay, when actually they only made two hort visit.
Although car park operators like to give out the aura that ANPR is infallible, it does actually have a high percentage of errors. This article from Parking Trend International reveals that although manufacturers claim 98% reliability, operators find they only get 90%-94% accuracy in real conditions, so as many as 1 in 10 plates are misread. In non-ideal conditions accuracy can fall as low as 60%.
The Parking Prankster