I can see forgetting about what happened two minutes ago in the cartoon or what color Black Vulcan's costume is supposed to be. Animators live life in the fast lane, and don't have time to double-check minor details like how many wings are on the batmobile. But how in the hell do not notice yourself drawing a third arm on a guy? Animator 1: "Me drew some arms on Green Lantern!" Animator 2: "How many?" Animator 1: "One... Two... oh look! A bunny!" Animator 2: "Let's chase it! We can count arms later!"