Continuity mistake: In an interior shot of the detectives' vehicle, the car is a late 80s Chrysler (LeBaron or K-Car), obvious from the 'pentastar' logo on the steering wheel and the distinctive gauge layout. When the detectives walk away from the car, it is the homicide standard issue Chevrolet Cavalier.
Other mistake: When the guest cast is listed and Jerry Orbach's name appears, Lennie Briscoe's first name is misspelled "Lenny."
Character mistake: In one episode, John, Meldrick and Tim are called down to their bank because they're getting a business loan to buy the Waterfront Bar (the bank's policy is to give good news in person). They start arguing and Tim announces at one point that "I've got a platinum card, Frank", which is marvelous, except he's talking to Meldrick.