In general, sequels to blockbuster films are expected to be inferior to the first, even if there have been notable exceptions to this "rule" over the years (The Dark Knight is superior in almost every way to Batman Begins, for instance). Having said that, most sequels are at least serviceable, despite the certainty of diminishing returns, because they already have an established story and characters to build on.
However, there are certain sequels that are so poor that they degrade or even kill a series with their mediocrity. The films listed below are all well-regarded in their own right, but they all suffered from sequels that entirely misinterpreted what made the originals so adored in the first place.
