The Pittsburgh Police badges are plastic resin casts of actual Pittsburgh PD badges. The props crew was given four badges to use, and thus all of the badges are one of only four numbers. This was done with the agreement that all casts and molds had to be returned to Pittsburgh PD so they could be destroyed once filming was completed.
When shooting their scenes as father and son, John Mahoney found himself without Bruce Willis when it came time to shoot his close ups as they had to play off each other. Mahoney had to shoot them with the script supervisor taking Willis' place.
On an appearance in May 2004 on "On the Record with Bob Costas (2001)," Bruce Willis apologized to the movie-going public for this film, saying that "it sucked," which is what Bruce Willis has typically done: say how terrific a film is when it's released, but then speak poorly of it later. He also said the same about Die Hard 2, 3, and 4 and Hudson Hawk.
Originally, the lead role was to be played by Ed Harris. It was then rewritten for Robert De Niro and, finally, Bruce Willis.
In the movie's credits, Bob The Cat is listed as Himself.