For a long time (maybe still?) it was common for police forces to buy cars which were keyed alike. What happened when those cars were gradually replaced? Well,

For a long time (maybe still?) it was common for police forces to buy cars which were keyed alike. What happened when those cars were gradually replaced? Well, the new cars had to be keyed to match the old ones, and taxis or anyone else in the market for old police cars would unwittingly be able to unlock the entire police fleet.