Revealing mistake: During the scene where the four Ninjas dressed in black have a pillow fight with the 'Girl' Ninja dressed in white, you can see that it's not the little girl fighting them, but a tall 'Oriental Man' stunt double instead.
Continuity mistake: After the dogcatcher, Mr. Cluge, takes Puppy Paws to the dog pound, he unfastens the dog's collar and hangs it on a peg on the wall. The next time the collar is shown, it is fastened again.
Factual error: At the beginning Scrooge is seen signing Jacob Marley's death certificate in the year 1836. English Birth, Marriage & Death certificates didn't come into being until 1837.
Plot hole: In the series we learn that these creatures take a lot longer than us humans to grow up and that they live a lot longer. They mention that the children are about 900 years old, so a grown-up would be more than 1000 (probably 2000). The series is set around our time (1990s/2000s) and Fjellrose and Erke are very old. Yet the film is set about 100 or 200 years ago, you can tell by the way the humans are dressed and the way their houses are built etc. So Fjellrose and Erke should still be grown up, but they are young children. If they are only 100 to 200 years younger, then they should be much older in the film or much younger in the series.
Plot hole: Mr. Cranston asks Denise to be the accompanist for the school's production of "Chicago". This is an error. In the year 2009, in which the film is set, this would be impossible since the licensing company which has the theatrical rights for "Chicago" restricts the show from being produced (even by amateur groups and schools) anywhere in the greater New York area while there is an open-ended production running on Broadway.
Character mistake: Near the end of the movie Godfrey says "I'll try to send the birds to the sea," but when he redirects the birds the operator says the birds are going back to the base.
Continuity mistake: The young 'au pair' does not have what it takes and resigns after falling into the fountain. As she speaks with the Caldwells, her wet hair is in front of her shoulders in a shot, in the reverse shot it is not, and so on. (00:04:20)