When playing on snapshot 14w25b, placing a block under Y=16 will make it invisible.

When I was working on my new map, I was trying to put some redstone mechanisms under the world so people wouldn't see. I went down and I noticed whenever I placed a block under Y = 16, it would turn invisible. It's really weird. I thought it might have been my mine craft glitching out because it is a new snapshot, so I restarted my game completely and it happened again. I checked it the block was a barrier block and it wasn't.

In order to perform it you have to.
1. Go down until you reach Y = 16.
2. Place any block under that cord.
3. You see your block invisible.