Trivia: Spoilers. In the end, Dr. Butler and his wife Stephanie are revealed to be the killers. They were actually the killers in the original script for the first film, but the ending was changed several times until the final version seen in theaters with Lori being the killer. The original idea was recycled into this film, since the dimension-hopping angle of the story allowed the presence of a new version of the killer.
Trivia: Arnold Schwarzenegger's "performance" in this movie "merited" a nomination in the 2021 Razzies (Raspberry Awards) for WORST supporting actor, but he lost [or would that be "won"?] to Rudy Giuliani! Arnold's nomination had some good company: Chevy Chase, Shia LaBeouf, and Bruce Willis. Bruce was nominated for THREE of his movies - Breach, Hard Kill, and Survive the Night.
Trivia: The Man as Panda had difficulty breathing after he got outside. He gasped for a while, then started to breathe through his mouth and was OK. The significance of this "cat turns into man" scene is that cats are "obligate nasal breathers" (automatically breathe through their nose) whereas humans can choose their nose or mouth. Cats can also breathe through their mouth, but it is a sign that something is wrong (overexertion, difficulty breathing, or other medical problem) = Get Cat to a Vet.
Trivia: When the screenwriter was pitching the film back in the 90's, reportedly at one meeting an executive suggested in all seriousness that Julia Roberts should play Harriet Tubman. When another person in the room mentioned that it wouldn't make any sense for the (very white) Roberts to play the role of Tubman, he responded "It was so long ago! Nobody would know the difference!"
Trivia: The Mermaid spit "ink" at Dr. Beyer in his car. This suggests a Mermaid is similar to an octopus, squid, or cuttlefish (which have "ink sacs" - though anal, not oral!) and is a type of mollusk, not fish. However, mollusks are invertebrates, and the Mermaid in the movie apparently had a spine/backbone. (01:13:28)
Trivia: The film features a cameo from Frank the pug, who appeared in the first two films, but was absent from "Men in Black 3." Oddly enough, despite only appearing in the film for a few seconds, Frank appears quite prominently on several posters and even on the DVD cover art for the film. (The same is true for the "worm guys," who also appear on posters and the DVD cover despite only having a brief cameo in the film).