What on earth is going on here? 

If you load a two-sided square prop - 2 polys, 8 verts - and render it, you get this off the wall pattern from the diffuse_color node. 
Same result and same pattern on both sides of the square.

However, if you use the alternate_diffuse node, it renders a complete square just like one would expect.

Whassup!!??

 
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