In Dustin Hoffman and Anne Bancroft’s first encounter in the hotel room, Bancroft did not know that Hoffman was going to grab her breast. Hoffman decided offscreen to do it, because it reminded him of schoolboys trying to nonchalantly grab girls’ breasts in the hall by pretending to put their jackets on. When Hoffman did it onscreen, director Mike Nichols began laughing loudly offscreen. Hoffman began to laugh as well, so rather than stop the scene, he turned away from the camera and walked to the wall. Hoffman banged his head on the wall, trying to stop laughing, and Nichols thought it was so funny, he left it in. 

(via nedhepburn)