Continuity mistake: When Edgar takes Duchess and the kittens to be abandoned in the countryside, he is on a motorcycle and sidecar combination. During the trip, the sidecar changes from one side of the bike to the other and back again. Also, when the sidecar separates from the motorcycle while the dogs are chasing Edgar, the wheel on the sidecar switches sides regularly as well.
Continuity mistake: About 3/4 through the film, which is supposedly set in the 1900s, there's an exterior shot of Jenny Agutter (as Roberta) talking to William Mervyn (as "the old gentleman"). Over Mervyn's shoulder can be seen a white delivery van driving rather fast along a road on the other side of the valley from where the filming took place (this is very obvious in the wide-screen version but less noticeable in the pan-and-scan TV version).
Continuity mistake: When the Winter Warlock's icy heart melts, he is holding a toy locomotive with its front showing, but the next shot shows the locomotive's rear showing.
Continuity mistake: When Cratchett gets the bird from the shop, it's clear that the bird is a duck. But throughout the entire film he's referred to as a goose.