Sometimes sequels answer questions the original movie left open or ambiguous. Sometimes they send a character in a direction that doesn't allow wiggle room. Even worse, some sequels retroactively change details from the earlier movie.
Bear in mind, in my opinion, a sequel can never truly invalidate an earlier movie. If I want to watch Halloween 4 as a movie in of itself, it doesn't matter that H20 negates its existence. Better yet, most people like to act like the later Halloween movies don't even happen. Personal Continuity reigns above all else. This is a thread, instead, designed to point out specific examples of in-story invalidation, and speculate on why the filmmakers went down that route.
For example, American Psycho 2 *spoilers ahoy*: truly a horrendous cash-in, but it's noteworthy in that it confirms Patrick Bateman's murders whereas the original movie left it ambiguous. Mila Kunis' character is, in fact, a victim of Patrick that manages to overpower him, kill him, and then later adopt his modus operandi. *end spoilers*
Another is the subtle, possible subplot in Poltergeist that Dana is pregnant. Whether or not that's what the filmmakers intended, Poltergeist II leaves no doubt that she wasn't, as it's a year later and Dana is now off at college. No mention of baby. The absence of Dana can be chalked up to Dominique Dunne's murder, and an illegitimate baby wouldn't have added anything to the plot.
