"One of my favorites is from Predator.

"Jesse Ventura was delighted to find out from the wardrobe department that his arms were 1" bigger than Arnold Schwarzenegger's. He suggested to Schwarzenegger that they measure arms, with the winner getting a bottle of champagne. Ventura lost because Schwarzenegger had told the wardrobe department to tell Ventura that his arms were bigger.""

"In Requiem for a Dream, there is a scene where Ellen Burnstyn gives a monologue about how terrible it is to grow old and useless (a scene credited for her Best Actress nomination). Unlike the rest of the movie where every shot is very carefully planned and framed, the camera here goes all shakey and off center.

This was not planned or intended, and director Darren Aronofsky started to throw a fit as soon as the scene was cut, because Ellen Burnstyn had just delivered an amazing performance and the cinematographer had screwed it all up.

As it turns out, the filming was screwed up because the cinematographer was so moved by Burnstyn's performance that he was crying uncontrollably - he couldn't hold the camera still, and couldn't see because his crying fogged up the eyepiece.

They used that cut in the final film. "