Unfortunately many visitors to Cambridge do not have special visors, so they think that there are no parking restrictions on the roads in Cambridge. They drive past the special invisible signs and park on the sides of roads with invisible parking restrictions which are conveniently close to the shops.
Parking attendants are issued with special books of tickets to use in invisible double-yellow line areas. These ticket books are also invisible, and they write on them with an invisible pen. Thus visitors often return to a vehicle with an invisible parking ticket attached, which they can't see.
If you don't pay the fine mandated by the invisible parking ticket within a fortnight then the council department which deals with such things sends out an invisible reminder letter. This is followed up a month later by an invisible court summons.
The first thing that most visitors to Cambridge know about is when the policemen turn up with invisible warrants…