Continuity mistake: When the everyone is singing and the Devil Robot is taking Bender down in the Lift, we can see that the back of the lift is grated in one close up shot, but in a far away shot it is a solid cube with no grate.
Continuity mistake: During the Robot Devil's song, Fry and Leela are going down a slide. At the beginning and end of their verse, they are shown as being on the same slide. While they are singing, they change positions several times.
As the power builds up in Bender, his left eye pops out, making him a cyclops, subsequently being eaten by the Cyclops eater. After the monster dies, Bender comes out, and has two eyes in the long shot, but puts his left eye back in the close up.