Why does stuff appear in my level when the exit door opens?

This happens when geometry from the level that follows yours overlaps with areas in your map. For example, if the level following yours has a pillar south of its entrance, that pillar will appear in your level, south of the exit.

This is easily avoided by ensuring there is nothing south of your entrances. If this is not possible, make sure that the level leading into it avoids all the geometry of the next level.