Wall of doors
Sometimes one side of a room consists of doors only.
Screenshot:
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###### Probable explanation: First, dungeon rooms are generated, and then an attempt is made to connect there rooms with corridors. However, when (for some reason) the generator creates a room with even numbers of rows/columns (normal rooms have odd numbers), the corridor generator doesn't take that into account and tries to connect to the room to the right-most/bottom-most tile - which would be rock. I assume that the generator generates corridor tiles before entering the room, and then digs throughout the room turning rock to doors until it hits a floor tile. Normally, it would turn only the first tile to a door - but when it hits the right-most/bottom-most row/column, which is actually the left/bottom wall of the room, it turns it all (or almost all) into doors.