Continuity mistake: When Georges enters Madame's room, the kittens' eyes can be seen all black, especially Toulouse's when the lawyer is dancing. A few scenes later, the kittens' eyes are back to normal.
Other mistake: Everyone stops when Roquefort shouts "Quiet!" to crack the lock, including Edgar. However, since the pets can't be understood by humans in real life or in the movie, Edgar should've been the only one to continue moving, and just the cats should've silenced.
Continuity mistake: When Thomas, Duchess, and the kittens run from the car driver after being spotted in the back, Thomas can be seen picking up Marie by the ribbon to carry her, but in the very next shot, Marie's walking on her own again. (00:37:41)
Continuity mistake: The small black cat first has blue eyes early in the film and then has light gray eyes further in the film.
Continuity mistake: When the alley cats leave the abandoned house with their broken instruments, it shows Billy Boss behind Scat Cat and Hit Cat behind him. When they are shown on the fence, their silhouettes show Hit Cat behind Scat Cat and Billy Boss behind him.
Continuity mistake: When the horse bites Edgar's coat-tails at the end of the film, in the next shot his coat is whole again - same error as when the dogs bite his pants.
Audio problem: While Thomas O'Malley is saying "all right now, don't panic" his mouth is not moving.
Continuity mistake: When the movie begins Adelaide's gloves are grey, except for the brief shot when she steps off the carriage and they turn cream coloured.
Continuity mistake: When Adelaide is about to step off the carriage the box has string around it. A shot later it swaps to a thick pink ribbon.
Continuity mistake: Just before O'Malley jumps on the "magic carpet" truck, it plainly shows it has no crank, but after it stops, a crank appears for the driver to restart it.
Continuity mistake: When the cats are on the piano during the "Everybody Wants To Be A Cat" sequence, there is a sheet of music in front of the pianist. When we see the piano and the cats falling through the floors, the music sheet has gone.
Chosen answer: It is the style of the artwork. Similarly, it is not a mistake that each individual hair is not drawn, the eyes are not really proportional to the head, etc.
Myridon