I found one of those skeleton spawning treasure rooms and noticed they were spawning in the wall. I got 3 pics of it with the walls replaced with glass to help see. It might be a little dark (needed to spawn them), sorry. You can make one out in the background in red. And another with one in the floor with the spawner.

Edit: Also spawning in full light & surrounded with glass completely.
Edit2: Still happening in 14w28b