Revealing mistake: When Rizzo falls down the chimney and lands on the goose at Bob Cratchit's house, he burns his feet. When he is next to Gonzo, his feet are smoking. If you look closely, you can see the holes in the puppet's feet where the smoke comes out.
Revealing mistake: During the ghost of Christmas present, Kermit is walking with tiny Tim on his shoulder and you can tell Kermit is walking on a spinning drum. Right before the camera pans away you can see Kermit's feet slide like he stopped walking but the road kept spinning. (00:55:00)