At Mark Van Meter's funeral, the minister says that Van Meter (a spy) was a success in the greeting card industry. In Get Smart (1965), Maxwell Smart was a spy whose cover was that he worked for a greeting card company.
During the scene in the police station after Terry was pushed into the river, there is an anti-drug poster in the background with the message "Speed Kills" and a picture of a person riding a syringe. This same poster was shown as being created during a 1967 Dragnet episode titled Narcotics DR16.
After principal photography finished, Penny Marshall wanted to shoot some pick-up shots in New York. She asked Gian-Carlo Coppola, her daughter Tracy Reiner's ex, to be the cameraman. Unfortunately, Coppola died in a boating accident before the filming date. The New York re-shoots were canceled, but Coppola got a 'special thanks' in the end credits.
The trivia items below may give away important plot points.
The phone booth sequence was part of a re-shoot in L.A. James Belushi, whose character was originally set to appear in two scenes, and die in a car accident in the second, was brought back, covered in bandages, to explain how he escaped death.