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.
Continuity mistake: In one scene O'Malley's tail tip changes from white to orange and to white again.
Deliberate mistake: I know this has been disregarded for the fun effect, but gasoline motors require air to work. When Edgar drives his tricycle into the river while being chased by Napoleon and Lafayette, it should have stalled and stopped working.
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: During the song 'Thomas O'Malley', Thomas jumps on the tree and shakes one of the branches to make the petals fall on top of Duchess. Just before they hit the ground, a whole bunch of petals appear on the ground out of no where and some of the petals that were falling to the ground disappear once they hit the ground.
Continuity mistake: There is a name plate above the cat door that appears and disappears.
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.
Continuity mistake: When Edgar locks the cats in the trunk, the padlock closes on the left. But when Roquefort unlocks it, it opens on the right.
Continuity mistake: Throughout the film, the studs on the collar Duchess is wearing vanish and reappear.
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