It happens once every blue moon, but it's actually rather easily solved. If someone else tries to go to that same world- say, looking for the person who got lost- they'll kind of dislodge the person who's stuck and wind up at the destination with them. Or sometimes that person will get set free by someone going to an unrelated world entirely. It's never happened to me and I've never had anyone appear, so this is just what I've read.

Oddly, people who get stuck between dimensions don't really remember much of what happened there. The accounts don't really say much.