Sometimes when placing half slabs down it'll be places half a block lower. If you're building a bridge and it happens you'll fall. It be first seen as connect to the other slabs then it will go down by half block. Its happened to me several times before and it sucks when I fall and die. I hope this can be soon fixed.
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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.