Uneven Distribution
So the system gives 4 false positives for every success, but it's OK because the Police will politely apologise.
However, in daily use those false positives won't be evenly distributed. There will be some people who look sufficiently similar to a suspect that they get stopped ALL THE TIME! Fortunately, sooner or later they will get so annoyed they will swear at or hit an officer, so they can then be safely arrested. Job Done, another dangerous person off the streets.
New crime: Impersonating a Criminal.