Recut: The Curse has the most stunning example of this. Apparently, the film ran over time and budget, so the suits decided to take it over to see how they could screw it up. Their version is the Theatrical Cut. When the film was shown on TV, someone got a hold of the now infamous Producer's Cut. While the violence and cursing were trimmed, an assload of alternate takes and different opening narration were shown, and the entire last 20 minutes of the film is RADICALLY different from the Theatrical Cut. The main change is that the explanation for Michael's killing ways is altered: The Theatrical version offered a scientific reason, but the Producer's Cut says the reason is supernatural (which also explains why Michael is also growing bigger in each previous film. It's because his power is growing). It also shows a final scene with Dr. Loomis realizing that he has been cursed by Thorn. This was likely altered when Donald Pleasance died. An early trailer showed that the film was originally going to called "Halloween 666: The Curse of Michael Myers." This version was only available through bootleg video releases until 2014 when it was included on the deluxe version of the Halloween Complete Collection Blu-ray set.
Series producer Moustapha Akkad had been intending to make a sixth Halloween film despite the tepid reception of Revenge, meeting with screenwriter and series super-fan Daniel Farrands in 1990. Farrands' ideas stoked Akkad's interest; he had compiled a notebook filled with research on the series, including a timeline, bios for every character, a "family tree" of the Myers and Strode families, and research on the runic symbol of Thorn that had appeared in Revenge. His intent was to bridge the first two films with the fourth and fifthnote For those wondering where the third film fits in: that film, Season of the Witch, was a non-canon spinoff that featured none of the series' characters and was subsequently ignored, viewed as something of a Black Sheep in the series today., and also to explain why series villain Michael Myers keeps coming back: he had been put under an ancient Celtic curse that compelled him to murder his entire family, one that would be passed on to another young child after he completed his task.
Farrands was brought on to write the film, but a series of complicated legal battles held up production for years until Miramax Films (via Dimension Films) bought the rights to Halloween. Writing finally began in 1994; several screenplays by different writers were gone through and deemed insufficient until Farrands' final draft, dubbed Halloween 666, was finalized after eleven drafts. From there came casting; while Donald Pleasence reprised his role as Dr. Loomis, Danielle Harris did not return as Jamie Lloyd, due to both salary disagreements and Creative Differences; Harris resented the fact that Jamie was to be killed off in the opening, feeling that her character was no longer important to the series, and so Jamie was recast. Fred Walton was tapped to direct, but dropped out and was replaced with Joe Chappelle.
Then production began, and the real problems hit. Shooting in Salt Lake City proved challenging due to an early winter that frequently interrupted production, and Chappelle and producer Paul Freeman had to rewrite the ending on the fly to meet deadlines. Furthermore, Freeman frequently inserted himself into production, rewriting dialogue and action scenes, removing a number of scenes from the script, taking it upon himself to direct second-unit shots, and sending the crew home when important scenes needed to be shot. Freeman's handling of the production was so inept that Miramax eventually stepped in, kicked him off the film, and ordered reshoots.
Post-production went no better. Lead actress Marianne Hagan described the test screenings in early 1995 as "consist[ing] primarily of 14-year-old boys" who disliked the ending and the Cult of Thorn storyline. This led to another round of reshoots to craft a new ending, but there was a big problem: Donald Pleasence could not be present for them on account of having died in February. Not only was a new ending shot anyway, but over twenty minutes of other footage was changed as well, leaving gaping plot holes that rendered the film nearly incomprehensible.
When it was released that September, Curse had the largest opening weekend out of the entire series but was ravaged by critics and fans, and plunged fast. One of its fiercest critics was Farrands, who hated the final film's deviations from his script. The series would be partially rebooted three years later in 1998 with Halloween H20: Twenty Years Later, which took only the first two films as canon. Eventually, when the film was shown on TV, someone unearthed the original Producer's Cut from before the reshoots; while it cuts the violence and profanity for TV airing, it otherwise retains most of the original content, and Farrands has given it his tepid (if still disappointed) approval. The full Producer's Cut was finally released on home video (after having been a popular bootleg for years) in 2014 as part of the collector's edition box set of the entire series.
What Could Have Been: Before the "Cult of Thorn" plot was fleshed out, the writers toyed with the idea of having the Man in Black be revealed to be Michael's father.
Donald Pleasence died during the shooting, so we never got to see his personal confrontation with Michael.
Not exactly true. The Producer's Cut of Halloween 6 was a finished version of the movie and the version producer Moustapha Akkad wanted to release (hence the name). Donald Pleasence successfully finished shooting of that film. It was only when the reshoots came about for what would become the Theatrical Cut that Pleasence had died.
Danielle Harris wanted to reprise her role as Jamie, and even had herself emancipated so she could work on it, but was turned down when the character's appearance was reduced, and she refused to work for scale.
Brian Andrews, who played the original Tommy Doyle in the first Halloween was considered for the role. However he didn't have an agent at the time and so couldn't be contacted. He later said he regretted it for years.
Denise Richards apparently auditioned for the role of Beth. Likewise the part of Dr Terence Wynn was written with Christopher Lee in mind.
As mentioned above, the original ending (seen in the Producer's Cut) has the Thorn curse passing to Loomis, with Michael, now presumably freed from his compulsion to kill, disappearing to parts unknown. Had the following sequels actually ran with this idea, the franchise's status quo could have been drastically altered.