It's not unfair to say that horror typically has a more liberal approach to continuity than most other movie genres, given the tendency for hit horror films to spawn a glut of sequels which contort the overarching mythology in increasingly credibility-straining ways.
And that's basically fine, though there are certainly instances of filmmakers introducing retcons to the franchise's lore which are soundly rejected by audiences at large.
There can be many, many reasons for why filmmakers would do this - perhaps they needed to come up with a way to keep the series chugging along for longer, or simply wanted to take the IP in a new direction, albeit at the cost of contradicting previously established facts.
Not all retcons are inherently bad, of course - sometimes they can help smooth over messy storytelling or even undo terrible prior decisions, but retcons are also certainly a tricky business, and the potential for leaving fans disappointed is colossal.
That's absolutely the case with these 10 horror movie retcons, which flipped the script in ways that hardcore and casual fans alike simply didn't vibe with...
10. Laurie Is Michael's Sister - Halloween II
It's no secret that everyone loathes Halloween: Resurrection's infamous retconning of Michael Myers' death, but the franchise's original sin retcon happened over 20 years earlier in Halloween II.
The sequel sees Michael (Dick Warlock) once again pursuing Laurie Strode (Jamie Lee Curtis) following her survival in the first film, but in an attempt to raise the stakes and give their conflict a more personal flavour, Michael's reason for pursuing Laurie is retconned.
Halloween II's big twist is that Laurie is Michael's younger sister, who was adopted by the Strodes after the death of her and Michael's parents.
It remains a hugely controversial retcon to this very day, because while the original Halloween implied that The Shape was a relentless force-of-nature driven purely by his need to kill, Halloween II gave him a more specific, even relatable motive, which in turn stripped away much of his terrifying ambiguity.
The sibling twist led to a glut of sequels centered around Michael hunting down Laurie and her bloodline, though this was itself retconned in 2018's Halloween film, which disregarded every prior sequel and stated that the sibling revelation was simply a rumour somebody made up.