As a corollary to this thread, how about movie scenes that made you feel as if the floor had been ripped out from under you.
Fight Club, when Edward Norton finds out about Tyler.
Deep Red, when we find out the painting was actually a mirror.
Henry Fool, when Henry is arrested- mostly what he was arrested for.
When we find out Vader was Luke’s father? 
When Neo awakens from the Matrix. [an accent]I damn near had a religious experience, I’ll tell you what![/an accent]
The end of Sixth Sense
Crying Game (ok, maybe not)
The death of Bambi’s mother (Shut up! I was only 6, ok?)
When Casper puts “one in the brain” of the Dane in “Miller’s Crossing”.
How about “the Gimp” of “Pulp Fiction”? Or perhaps what Zed and his crony did to Marsallas.
Those damn flying monkeys freaked me out for a long time too!
The end of “The Game.” Blew me away!
-p.
The last ten minutes of “Seven.” Wow.
The Exorcist - When Father Karras commands the demon to “come into him” and then witnessing the reaction on his face as he is possessed. ::shudder::
Executive Decision - Steven Segal dies very early in the movie when the high tech plane disintegrates.
Predator - The entire sequence from when Jesse Ventura gets his torso blown open to Bill Duke picking up the mini gun and “mowing the trees down” and then everybody else joins in with their firepower.
Fellowship of the Ring - My wife (who’s never read the books) was stunned when Gandalf fell to his “death”.
Hannibal - When Lecter lifts the top of Ray Liotta’s skull very nonchalantly.
Jurrasic Park - On first viewing I was as stunned as the characters on screen when seeing the first dinosaur (I think it was a brachiosaurus). First convincing CGI that I had seen.
Robocop - When Peter Weller gets shot up by the bad guys in the beginning. I still cringe when I watch his hand get shot off.
I’ll second the ending of Sixth Sense and the brachiosauri grazing scene from Jurassic Park. Also…
–Y tu mamá también, when you find out why Luisa took the road trip.
–a good portion of Les amants criminels, but especially the ending.
–The bus crash/train wreck scene from The Fugitive, as well as the jump off the dam.
I was totally shocked by the ending of The Grifters, with John Cusack, Annette Bening and Anjelica Houston. It totally came from out of nowhere and stayed with me for some time afterwards. Chilling!!!
The last scene of No way Out.
Sam L. Jackson’s unexpected departure in “Deep Blue Sea”…Timmy
Smooth-talkin’ Jack Vincennes (Kevin Spacey) meets his end in L.A. Confidential
The, er, unusual precipitation near the end of Magnolia
The Usual Suspects, when Chazz Palmintieri starts looking around his office after letting Kevin Spacey go and realizes he (and the audience) has been had
Oh, yeah… Magnolia. I remember the collective gasp that erupted from the audience at that part.
The end of “The Others” starring Nicole Kidman. I was so sure I know what was going on – suffice it to say, I was dead wrong.
.:Nichol:.
The end (uh, beginning?) of Memento.
Fifteen Iguana
Man, I was on the verge of having this feeling towards the end/middle of Vanilla Sky. But the film just couldn’t pull it off. Very disappointing.
If we’re talking scenes that floored me as in “God Damn, that’s amazing”, then I’d have to nominate pretty much every scene in Hard Boiled. If we’re talking “extreme plot twist”, then I’d put up “Arlington Road”.
Hands down, just about any scene from “Sleepers.” The scene where the boys were taken into the room with all of the crates, man… I still remember that years later. One of the other scenes that absolutely shocked me was when the little boy was saying the rosary and the guard comes up… I can’t believe people could actually do this sort of thing, and the way it’s depicted in the film is just heart-wrenching…
Floor-dropping scenes? Try completely forgot the damn floor was even there!