I can confirm this is indeed a bug. I tried reporting to Minecraftforum, but they sent me here. I can also confirm it has happened with trapdoors as well. I'm not trying to promote anything, but you can see for yourself here:http://www.minecraftforum.net/forums/support/unmodified-minecraft-client/2471818-very-weird-half-slab-glitchIt's a very unfortunate bug, one I truly hope gets resolved.
user-8cde1
I can confirm this is indeed a bug. I tried reporting to Minecraftforum, but they sent me here. I can also confirm it has happened with trapdoors as well. I'm not trying to promote anything, but you can see for yourself here:
http://www.minecraftforum.net/forums/support/unmodified-minecraft-client/2471818-very-weird-half-slab-glitch
It's a very unfortunate bug, one I truly hope gets resolved.