3. Poltergeist (1985)
The "False" Ending... When their daughter Carol Anne is captured by ancient spirits and sucked into another dimension, the Freeling family must do whatever it takes to get her back. This eventually has the mother of the family, Diane, going through a portal into another world to claim her. Though they face off against a number of horrifying spirits and manifestations in the process, Carol Anne is rescued, and we're told their haunted house is now "clean."
But Wait...Poltergeist could have wrapped up here and remained a classic horror flick, though director Tobe Hooper hasn't had his way with us yet. Though at this point it genuinely feels like a logical place for the story to end, complete with the family preparing to move out, we're treated - during the family's last night in the house - to another round of spirit attacks. They survive, of course, and the Freeling home is sucked into a vortex and vanishes, whilst the family opt to spend the night in a Holiday Inn. It's a surprising occurrence, but it totally pays off.