When you place a full block on any lower half slab the full block is transparent.
Edit: This is an Ocean/Deep Ocean Biome bug only. Other Biomes seem ok.
Single Player Creative tested 14w25b
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Could not reproduce (screenshots attached).

Did you try over an ocean? I didn't specify, but the pics clearly show an ocean biome. I'll test other biomes too. Leave f3 on in your screenshots to verify version as well.
It is an "Over Water" thing. This is related to the post with post about blocks losing their texture over an ocean... Notice the top of the dirt block is missing. (transparent4.png)
In "Biome Based.png" it clearly shows the biome matters. I had tried to put a sign on the invisible block but it was invisible too.
Tiny bit more troubleshooting: Everything between y:64 and y:96 is transparent, at under y: 63 and above y: 96 the blocks are normal.