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.
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