When Perdita and Pongo are nuzzling after the news that Cruella is not going to get any of their puppies, before it changes to the scene to reveal the puppies nursing, the tags on Perditaand Pongo's collars vanish.
Cruella's phone when it was first picked up (to be Jasper), has a face of a devil with its eyes open and wide-mouth smiling. Yet when she hangs up and calls Anita, the face has its eyes closed and is heavily frowning.
After Cruella de Vil leaves the house after looking for the puppies that haven't been born yet, Roger comes from the attic singing the Cruella de Vil theme, holding a long cigarette. When he reaches the bottom of the stairs, the cigarette vanishes.
When Sgt. Tibs first enters Hell Hall, we see the main hall. The stairway is to the left, it goes up to a landing, then turns right. When the Baduns chase Tibs and the puppies, the hall has been mirrored - the stairs are on the opposite side of the hallway, and they turn left.
When Jasper reaches for the bottle to throw at the escaping cat it still has wine in it, but when the bottle breaks against the wall, the bottle is empty and no wine spills out when the bottle shatters.
After the art lady in the attic yells "Prissy, come in here!", she pulls Prissy inside and the window closes. In the next shot when Prissy knocks over the painting as she runs over to the other side of the attic, the window she was at is opened again.
When Pongo opens the clock to change the time to 5:18, for his evening constitutional, he starts to bark, letting Roger know its time for his walk. But he forgot to close the clock door. But when Roger looks up, its mysteriously closed.
Patch's right ear is solid black, and his left ear spotted. However, sometimes the left ear is solid white, and sometimes it's solid black. In the same scene where Patch turns into Lucky, his ear goes from spotted to white, and in the following scene in the Colonel's barn, both of his ears are solid black.
In the beginning, Roger is shown to wear a wrist-watch which disappears when he first lights his pipe, and reappears in the next scene. When he goes to the park assuming it was hidden under his coat sleeve, it is gone when he takes off his coat to give it to a wet Anita.
During the Twilight Bark, there is a scene showing St Paul's Cathedral across the other side of the River Thames. This means that the view must be facing north. However there is a full Moon near the horizon in the background, which would not be seen in a north-facing view.
Pongo and Perdita set off from London to Suffolk (north), but when dawn breaks we see them running left to right across the screen, with the dawn breaking in the background. Since dawn is in the southeast in England in the winter, this means they are running roughly southwest.
When Pongo starts howling to send out the last part of the Twilight Bark, his four paws temporarily do not make contact with the ground; giving the image of walking on thin air before being pulled back to the ground by his owner.
In the title screen, you can see that all of the pictures are different poses of Pongo. But looking closer, some are clearly designed to be Perdita, especially when in the same pose together, when she was designed/spotted to be Pongo; despite the slimmer body.