In the scene where we meet Carl Fredricksen as an old man, he unlocks the door to go outside and sit on his porch. The audience sees the outside of the door as Carl unlocks the door - and we see 4 keyholes for the locks.

After he finally unlocks all the locks, including the slide chain that is stuck the first time, he comes outside and shuts the door. When he shuts the door however, there are only 3 keyholes for the locks. For the remainder of the movie, there are only 3 keyholes on the door!