Crank 2 High Voltage.
Big let down in my opinion the first crank was awesome. The second one was not so good. The second one just kinda looked like kids directed it the graphics were awful and it really made no sense to me.
Any movie that gets really good and then the ending just makes you mad because of the constant overuse of everything having a happy ending.
One movie in particular I remember is The Darkest Hour. You make a movie about light/energy-looking aliens disintegrating pretty much anything living that they can sense, even going as far as having someone's mistake causing them to kill the only guy who's managed to make weapons to defend against them, only to have them randomly sailing away in a submarine or whatever at the end. Seriously?
It seems like a lot of movie makers just slap on happy endings to make you feel good, no matter how ridiculous they are.
That's with a lot of sequels, although I like the Pirate ones. Most of them can't compare to the originals or it seems that they just try too hard to make them better or just milk the franchise.