There are a couple that jump to mind. One was in A Simple Plan where a lady was shot and flew back about 15 feet in the air. It looked so unnatural and something out of Sam Raimi's other films, I lost it!
Another scene was in Conspiracy Theory when Mel Gibson was trying to escape being tied in the wheel chair. He was rolling down the steps and knocked one of the bad guys out the window and yelled "he's flying."
I hate to say this, but when Mickey dies in ROCKY III. Burgess Meredith's acting is fine in this scene(as it almost always is) but when Sly Stallone starts howling like a banshee I lose it every time. Also, that scene in the HAUNTING remake when the guy loses his head. The audience I saw that with erupted with laughter at that scene.