Continuity mistake: When Don Knotts is in the old Simmons mansion, he falls down a chute of some kind. If you watch closely you will see the stunt person looks nothing like Don Knotts. It is a younger man with very straight hair.
Continuity mistake: When the Batmobile is going to park (before the bomb scene) there is a sign saying "Mobey Dick Parking Only". When he is later carrying the bomb and passes by the Batmobile the sign is not there.
Continuity mistake: At the end when the sled full of Christmas stuff is on the mountaintop, it starts sliding off the edge. It's going fast enough that it should be long gone by the time the Grinch reaches it, but when he does it's still right near the top of the mountain.
Other mistake: When Snoopy starts sneaking through the pumpkin patch, Sally is standing with her arms crossed and Sally's hand is painted blue while talking to Linus. (00:20:35)
Continuity mistake: Throughout the movie Elsa wears a collar. In the scene where she runs away the collar is visibly in place - a few minutes later, when she's found with the elephant, the collar has disappeared.
Revealing mistake: All four of the Dachshunds in this story are supposed to be female. During one part of the picture, when they jump on the bed with Suzanne, you can see that at least one is male.
Revealing mistake: Actress Yvonne deCarlo's palms and insides of her fingers do not have her green makeup applied like on her face, arms, etc.
Continuity mistake: When the four kids uncouple the coach from the engine Carole is wearing trousers. The stationmaster tells them to stand clear as the engine pulls away, in the first shot of the children after the engine has gone Carole is still wearing her trousers but in the second shot Carole is now wearing a skirt.