In the Conservatory of the Disneyland Haunted Mansion, directly across from the coffin, on the other side of the tracks, you'll find a door, and to the left of the door, you'll find this:
been there, at any rate. You can glimpse it in the Osmonds special from March 1970:
The answer is probably something mundane. Perhaps the door frame came out of the mold defectively, and there was no time or money to cast another one. (Like other HM fixtures, it's fiberglass.) So they had to improvise. There is another possibility, however, and this is admittedly speculation.
One effect the Imagineers were reportedly contemplating for the Corridor of Doors was a "girl in the mirror" gag. The following piece of concept art comes from a later time, but I understand that the effect it shows was kicked around by the original Imagineers as well:
It is a tribute to the Haunted Mansion (and to the geekiness of its fans, I suppose) that a trivial find like this one engages the imagination. I doubt that a missing door in It's a Small World would get blogged anywhere. But the Mansion Imagineers have created an environment that encourages you to enjoy the illusion of a house that's more than a house, one that's almost alive. Looking at some concept art, you can see just how far Imagineers like Claude Coats were willing to push the surreal nature of the building itself.
So for those who are happy to absorb the atmosphere of the place, the missing door becomes something intriguing once you're aware of it. You could almost think it may appear someday, or better, that it is there right now but you can't see it. *cue the eerie organ music*