I had an idea for an extremely awful town. Imagine:

You're traveling through the woods. As you walk along the dark path, you come to a clearing. Upon reaching this clearing you can see a large town of about two thousand. The church bells are ringing, and you can hear the organ playing. As soon as mass ends, the townspeople walk out, and go about their lives. You go to the blacksmith, the tavern, the inn. It seems like a normal town. When you wake up, the inkeeper is a different person. You ask where the other inkeeper went. You're told that he went out into the woods. You shrug it off and go to see the blacksmith, who is also a different person. In fact, none of the people here are the same as last night. The entire town has changed, overnight, and nobody seems to remember the events of the day before. The church bells are ringing. It's time to go to midday mass...

The entire town is populated by changelings, and none of them know that the others are changelings. They don't even know that they are changelings themselves.