Update: Specifically, this is Block 36. Right-clicking on the block makes it disappear.

An few invisible blocks have appeared in my multiplayer world. They obey specific rules:
1) They do not inhibit movement.
2) They do not prevent a block from being placed in their voxel.
3) They can be picked, but only if the test for a non-invisible block fails. Specifically, a neighboring block to the invisible block can be picked when hovered over, but when hovering over empty space, the empty voxel is selected.
4) A few are shifted by .5 blocks up .
5) When breaking is attempted, wooden particles appear.
6) Pistons are unable to push them.