Board Thread:Administrator's Noticeboard/@comment-12069596-20131129171259/@comment-7697305-20131130185047

That is almost certainly a bug, and it looks like one that was introduced by a recent update: http://www.terrariaonline.com/threads/18-glass-blocks-to-make-4-glass-walls.121383/

I imagine that it will be fixed at the next update.