4. Signs
The Ending: It transpires that the alien invaders are in fact allergic to H2O, and that fact essentially causes their invasion to completely fail.
Why It's Infuriating: Even ignoring the fact that it's another laboured twist from the M. Night Shyamalan stable, it's a shameless rip-off of War of the Worlds' "nature saved us" twist, and given how seemingly advanced these aliens are, it's patently idiotic that they didn't scope out Earth's atmosphere before invading. It sacrifices a coherent ending for an ironic one that didn't so much thrill audiences as have them groaning all the way home.