Other mistake: Towards the end of the movie, when Superman drops Gus off at the coal mine, Gus begins talking with the coal miners about him and Superman. He then rips open his shirt (as if he had a Superman costume on underneath). How did Gus know that Clark Kent tore his shirt like that in order to change into Superman, or keeps his suit under his clothes at all? The only people who know this fact are the ones who have seen the transformation from Clark to Superman, i.e. the audience.
Other mistake: DVD and Blu-ray only: in the Battle of Endor, right after the rebel fighter says "there's too many of them." The Millennium Falcon goes into 1/4 of the picture, you can see a black square spot on the right side of the flash frame due to a bad filter usage. Widescreen only. It does not appear in the 4K release even though it wasn't removed.
Other mistake: In the marching up and down the square skit, the guy behind Eric Idle is trying not to laugh. (00:33:35 - 00:34:05)
Other mistake: Scrooge drops several gold coins into Mole's hat and places the hat on Mole's head. When Mole removes the hat, there are at least 40 coins but Rat exclaims, "Twenty gold sovereigns!" (00:00:22)