Deliberate mistake: You might be wondering where Terry suddenly comes up with Jolly Jack all of the sudden after escaping the lab. In the commentary, they say that the Joker, in his transmission to Terry in the lab, was supposed to be eating out of a candy jar with "Jolly Jack" on it, but couldn't have it due to time constraints.
Trivia: When the film was initially released, there was still ongoing backlash against violence in media (particularly media aimed at children/teens) as a result of the Columbine school shooting. Warner Bros. got cold feet and had the film edited and new alternate scenes written and animated to cut down on the violence on-screen. (Most notably in the "Death of Joker" flashback which not only had Joker dying in a different manner, but had a sequence cut showing Joker stabbing Batman in his leg, giving him his limp) Warner Bros. Initially did not wish to release the original version of the film, but gave in after fans petitioned to see the original intended version.