Desert Fossil Wall

No world generates with this behind its fossil patches, so tunneling through an underground desert hoping to harvest some is wasted effort. Every piece in the game came off a crafting station, built out of Desert Fossil. It was clearly meant to work the other way: the wall sat in the game for several updates with no source at all before crafting was added, which reads like planned generation for the underground desert that never got finished.

There are two of these in the code. The one you place is the later addition and behaves like any player built wall: nothing spawns behind it and it counts toward a valid house. The original is still in there, unused, flagged unsafe, and it does not even drop an item when you break it. Nothing in normal play will ever put that version into your world, so this only matters if you are editing a map with outside tools and cannot work out why a wall you painted in refuses to stop spawns.