Drilled too deep into a thin wall, how can I fix that?
Cut away any burs or rough edges around the unwanted hole. clean out any dust or loose particles.
fill with filler to a level below the surface of the wall (ie. indented) allow the filler to set hard.
Fill again below the surface of the wall, allow the filler to set hard.
fill again level with the surface allow the filler to set hard.
fill again slightly proud, allow to set hard,  sand until flat.
paint to match, 3 coats.