Patch in One Hundred and One Dalmatians (1961) had his black ear and patch on the same side. In this film, they are on opposite sides.
(at around 54 mins) While in the cage Patch talks to another pup he refers to as 'Lucky'. Lucky in One Hundred and One Dalmatians (1961) had two black ears, while this pup has white ears.
(at around 2 mins) In the beginning of the movie during the "I See Spots" segment, when Roger is standing in the front of the piano, playing around with the keys and holding one of the puppies, his chair has moved and can barely be seen behind a box and is closer to the wall. Then when Roger comes up with the "seeing spots" idea for his song, the chair is back in front of the piano.
In the beginning, Roger's piano is a console. Then later on when he's playing his piano in the basement of the new home, the piano is an upright.
(at around 52 mins) Shortly after Lil' Lightning left the building and it was starting to rain, you can see rain drops on the glass windows of the roof but rain wasn't shown falling inside the building from an open window.
When Jasper steals the truck, the wheel is on the left side, which is the wrong side in the film's setting in the UK.
Both Pongo and Patch incorrectly state that there are 101 puppies. Actually there are only 99 puppies. Cruella De Ville, Jasper and Horace state this correctly.
Jasper says Horace's line "Poison 'em", and Horace says Jasper's line "Drown 'em", at the point where Cruella asks Lars to make the puppies into a revolting art project.