John Belushi filmed a cameo "guest star" appearance for the "Testimony of Evil" episode, showing him underwater wearing a pair of cement shoes. He died before the episode aired, so a new sequence was filmed with William Conrad.
After the success of the "Naked Gun!" movies, both Comedy Central and CBS-TV, and not ABC-TV, re-aired all six episodes of this series in the summer of 1991. Producers wanted to include the John Belushi "guest star" appearance in the rerun of the "Testimony of Evil", which had been cut from the episode due to Belushi's death in 1982. The footage could not be located, and is now presumed to have been lost or destroyed.
Drebin's rank is oft intentionally misused, i.e. he might announce himself as 'Lieutenant Drebin' only to have that person immediately introduce him as 'Captain Drebin.' Even Drebin varies his title, making it one of those distinctively amusing subtleties the Abrahms and Zucker Brothers team are known for.
Starting with the second episode, Drebin would reel off a list of the criminals apprehended in all the previous episodes. This joke was slightly ruined, as the episodes aired out of order, the fifth episode airing before the fourth and the fourth airing before the third, thus criminals were referred to before they were arrested. This joke was not done in the first or third episodes.
According to Pat Proft, had a second season been commissioned, the spot in the opening credits where Abraham Lincoln is shot at and retaliates with a pistol would have been replaced by a similar one, featuring Mohandas K. Gandhi and a machine gun. This joke was later used in a fake trailer for "Gandhi II" in 'Weird Al' Yankovic's movie UHF (1989).
Although none of the "Special Guest Stars" appeared in any of the episodes of the series due to being killed off during the opening sequence. One guest star (Robert Goulet) DID star in The Naked Gun 2 1/2: The Smell of Fear (1991), one of three movies based on the series.
The original video release of the series (before DVD) seems not to have obtained the rights to all the songs used in the original show. Songs such as "Happy Birthday" (that are copyrighted and therefore require a royalty) were replaced with alternates that were presumably royalty-free.
In 1994, Leslie Nielsen (Frank Drebin) appeared in the popular British Saturday night entertainment show "Noel's House Party". Which at the end of the show, Nielsen and host Noel Edmonds parodied the famous closing credits from "Police Squad!" during the closing credits, which Nielsen and Edmonds faked a freeze frame, which the cast did in the closing credits of each episode.
The episodes end with a mock freeze frame. However, instead of an actual freeze frame, the characters in the frame simply stand as still as possible. In one episode, the frame "freezes" with one character pouring the other a cup of coffee. As they stand still, the cup can be seen overflowing.
Each week Frank's snitch, Johnny the Shoeshine boy who knew everything, would meet with someone else that Johnny could give useful if esoteric information to. In the last three episodes he met with real-life people Dick Clark, Joyce Brothers, and Tommy Lasorda (even providing Clark with a special bottle of anti-aging face cream).
Peter Lupus, who played the recurring character of Nordberg, was replaced by O.J. Simpson in the "Naked Gun" motion picture series that derived from this television series. Alan North, who had played Capt. Ed Hocken, was likewise replaced in the motion pictures by George Kennedy. Although Lupus had had a run on the series "Mission: Impossible," it's likely that both North and Lupus were considered too little-known as actors for a motion picture series, and therefore were replaced by the more bankable Simpson and Kennedy.