Franchises. Most genre's (action, comedy, even drama) have them. However, for ever "A New Hope", there's "The Phantom Menace", and as horror fans know, that's no exception within that genre. So let's take a look at when horror franchises either jumped the shark or went totally off their rockers.
Jaws: I'm going to skip the obvious pun and mention that while the prior sequels weren't good, "Jaws: The Revenge" both went insane and sucked hard. On paper, the idea of a shark roaring, Michael Cain working simply for a paycheck and a shark with the basic concept of revenge is amusing. In execution however, it's mostly just bad, and in the end killed off the possibility of any more sequels. It didn't however, kill off shark movies, which we still get to this day.
George Romero's "Dead" Series: It had a good run, but by the time "Diary of the Dead" came out, something happened: George Romero's horror with a social conscience had started to run dry. It was at least better than "Survival of the Dead", which featured bad jokes and managed to do nothing to further advance the series or say anything interesting.
Friday the 13th: After "The Final Chapter" made a bunch of money, a sequel was made-even though the last film was supposed to be just that-the last entry. What viewers got instead was a guy pretending to be Jason Voorhees, and lots of really bad moments. While "Jason Lives" restored faith, we then got an okay sequel, a bad one, and then "Jason Goes to Hell", which serves as the worst entry in the series, in which people are possessed by Jason. Then there was "Jason X", which was an improvement, but also gave us Jason in space as a killer cyborg. Not a terrible movie ("We love premarital sex!"), but the premise itself is rather bizarre.
A Nightmare on Elm Street: "Freddy's Dead: The Final Chapter" more resembles a really bad sitcom turned full length movie than it does a horror movie, with pointless cameos (Roseanne Barr and Tom Arnold), a bad attempt at providing back story, and Freddy being reduced to a prancing clown. The remake is nearly as bad, with "Freddy is a child molester" being ramped up to the point of it being gross, and more plot holes, lapses in logic and bad directorial decisions than you can count.
Halloween: By the time "The Curse of Michael Myers" came around, we were introduced to the Cult of Thorn, and Michael having something to do with a cult to me at least was insulting, as he didn't need that kind of thing. Then there was "Halloween: Resurrection", in which Busta Rhymes beats up Michael, and Michael kills his sister Laurie, which essentially destroys the entire point of the series. Then Rob Zombie remade it and did a sequel, which in turn gave us laughable moments (the use of the Nazareth song "Love Hurts"), obnoxious symbolism and white trash stereotypes.
Hellraiser: While "Hellraiser III" threatened to jump the shark with it's goofy Cenobites and bad puns, "Hellraiser: Bloodline" gave us the Cenobites in space (though it at least gave us explanations for the device) and was done by two directors-and the end result was given the infamous "Alan Smithee" pseudonym. The eighth film "Hellworld" introduced Pinhead and the gang to the world wide web, and if you've seen "Halloween: Resurrection", you know how that works out. Oh, and "Revelations" didn't even bother to bring Doug Bradley back as Pinhead.
Scream: I'm a big fan of "Scream", and the second one, while not as good, still has some choice moments. The third one however, sees the joke being stretched to it's limit-and oh God, that awful soundtrack with the likes of Creed, Dope and Coal Chamber! The fourth one was better, but at the same time, it had a conclusion I didn't care for and ultimately felt dated.
Leprechaun: To be honest, this series never did jump the shark. They were movies that came in a time in which horror wasn't as profitable and weren't any good to begin with. By the time "Leprechaun IV" came out, he went to space, though the movie is almost worth watching for the titular creature jumping out of a man's penis and a woman who loves to take her clothes off. Then there's the "Leprechaun in the Hood", which was a punchline before it came out.
Sadly, we never got something like "Next Friday the 13th." Come on, the idea of Ice Cube fighting Jason is awesome.