In the scene right after Hal got the promotion him and Rosemary are in his office and his co-workers come in with cake. As Rosemary leaves she picks up a knife and takes a huge chunk of the cake and sets the knife down on the plate and leaves. The shot then goes to Hal, and then back to the people with the cake and if you look at the plate the knife has gone and changed places. The shot goes back to Hal for a minute and then back to them again. And what? The knife has moved itself into another position yet again.