I was looking for a Nether Fortress in Snapshot 13w39b (at least that's what I generated my Nether in) and ran across a floating item frame with redstone in it over a lava lake. The item frame cannot be broken nor can the redstone dust be removed. If I had to guess this is meant to be a terrain generation place holder for a Nether Fortress, but for obvious reasons I can be certain.
I'm including an imgur link with screenshots of the issue.
Coordinate wise - it's at roughly x212, y53, z69 in the Nether.