Other mistake: When the nanny comes home with the tags, they call Jewel to get hers first. Jewel has a penis.
Continuity mistake: Shortly after Jasper & Horace steal the puppies, a blue Renault 4 is seen going past the entrance to the mews. About 3 seconds later it goes past again in the same direction.
Continuity mistake: When the puppies are watching the television, you can see that they are watching Homeward Bound. The three animals are standing on the hill, about to head home. A few minutes later, when the nanny comes inside, we see the TV again and Shadow is with Peter. This happens at least 10 minutes after the animals are on the hill.
Revealing mistake: In the mansion where the two flunkies are chasing the puppies that are to be skinned, one of them flies off the roof, through the stone battlements and crashes into a frozen pond. You can clearly see the stonework landing around him in the pond and bouncing/floating.
Audio problem: When Jasper and Horace answer the door at the DeVil Mansion, Jasper says "Good eve-ning" as he opens the door. But as he says "-ning", the camera shows a front shot of the pair of them, and Jasper's lips aren't moving.
Other mistake: When the two robbers are following the puppy trails through the woods and are nearing exhaustion they spot the police convoy coming along the road. The cars appear to be mostly Range Rovers. In a latter scene though, Cruella joins them in the prison van and the convoy of police cars is seen to be leaving the village, but most of the police cars appear to have changed to Land Rover Discoveries, instead of the earlier Range Rovers.