OK, I changed my command blocks to that format.
This happens with sandstone slabs, cobblestone slabs, stone brick slabs, nether brick slabs, dark oak slabs, and dark oak leaves!
Usually after a while, Minecraft undergoes an intense amount of lag (<1 fps) while trying to load all the chunks, but fails.
Try changing your render distance - it worked for me! However, it may still happen later.
By the way, this is in 14w26c.
This happened to me while I was playing Hunger Games. I ate a piece of food, then scrolled to my sword. Then I ATE MY SWORD.
I guess that's what Mojang meant by bending time around space instead of space around time.