There's been lots of discussion lately about when particular science fiction movie series jumped the shark. Jay Garmon at Sci-fi rant says the Star Trek series jumped it with the First Contact movie, while with Star War it was the Midi-Chlorians of The Phantom Menace (meaning the Jedi knights are merely a master race, not meditation masters in touch with the greater universe). I agree with Jay about the Star Trek movies--First Contact was where I lost interest in the series. But with Star Wars, I believe John at SF Signal hits the nail on the head by saying that the true shark jumping was with the Ewoks. Evidently Lucas had originally planned for the Return of the Jedi to focus on Wookies, but for merchandising reasons made a script change to "cute, cuddly Ewok toys." The rest, as they say, was shark jumping history.
In my opinion, other SF/F cinema shark jumping includes the killing off of all the best characters at the start of Alien 3. I mean, hey, when you have a great character like the android Bishop (played so perfectly by Lance Henriksen), why wouldn't the first thought of any director be "How can I get rid of this SOB?" Another shark jumping moment is the Wachowski brothers believing that their hip look at religious themes in the first Matrix movie was worth total immersion in philosophical babble during the next two films. The shark finally ate the Wachowskis when Neo was crucified in The Matrix Revolutions. After all, if you're going to jump the shark, there's no need to be subtle about your intentions.