All through the movie are scenes that involve the outside door of the house where our heroes live... this door has a li'l hatch for the kittens to freely move in or out...

So what, you'll say...

Well... the door's panels' number (left to right) switches. First there's three, with the hatch in the middle. Then, after the scene where Edgar has put the kitten to sleep, there's four... and in the scene right after that, we see Edgar sticking his head out of the hatch... which is in the rightmost panel... and the leftmost is next to it. (This means there's only two panels!)