Every now and then, a movie will accidentally start a franchise. Usually, this is the result of an original script being grafted onto an existing premise, like how Saw 2 was an unrelated script called The Desperate, or how American Psycho 2 started production as a standalone story before some Patrick Bateman references were crammed in.
An especially strange example of this phenomenon is 1992’s Bad Lieutenant, starring Harvey Keitel and directed by Abel Ferrara. It was followed by 2009’s Bad Lieutenant: Port of Call New Orleans, which was kind of a very loose remake of a movie that director Werner Herzog said he had no interest in ever seeing, prompting Ferrara to express his desire for Herzog and star Nicolas Cage to die in a streetcar explosion.
