Revealing mistake: When the nanny comes home with the tags, they call Jewel to get hers first. Jewel has a penis.
Other mistake: When Cruella gets angry after offering the couple £7000 for the puppies and they refuse the offer, she storms out of their house and heads for the car. She then hits one of the two stooges and as she goes to get in the car you can clearly hear her say the word "fu*k".
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.
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: In the scene where 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 Discovery's, instead of the earlier Range Rovers.
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.