The condition that many sequels to successful films suffer from: re-hashing of plot points, shoving the best parts of the original back in your face until you almost hate it, ETC, all while trying to make the movie "bigger and better" than the original through the addition of an over-abundance of special effects, or new characters that you just dont care about (or, on some occasions, omitting major characters that people DID care about). Sequelitis can make one of 2 things happen: the third film will back off and be much better than the second, or part 2 will have been so awful that a 3rd never even gets made.

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A medical condition propogated by a combination of commercial success and creative ineptitude. Symptoms include bad movies and music, arrogance, denial, and desperation. In that order.

Treat symptoms with a daily dose of self-realization.

A case in which a film/video game/book is popular when it is released, however, it's popularity or general enjoyability decreases when the sequel is released. Sequelitis is particularly common with video games as a sequel may have improved graphics, more intuitive controls, etc.