Board Thread:General Discussion/@comment-29546017-20160809202024/@comment-27701762-20160809202850

Toriel says that the door is a "one-way exit." So either A) Toby screwed up with the endings, as Toriel is able to return to the Ruins (and Sans and Papyrus are able to accompany her), B) the door can only be opened by powerful magic possessed by the Boss Monsters, so that while Toriel can go back and forth, it is effectively a one-way exit for anyone else, or C) the door is indeed a one-way exit, but there is an entrance somewhere back to the Ruins.