Material Preview Final Node vs No Preview = Different Results


Sorry for the confusing title, it’s past 5am and I’ve been at this for forever… so basically, without going into man specifics, this is my material setup (real dirty):

Now, you’ll notice that the preview is pitch black. However, if I hit preview on the Frag node, I get this:

How is this possible? Frag is the last node in the network. There is no change if I shift the UV1 and UV2 channels up, I use those to transfer color values, not UV values. In the screenshots a WorldPos node is plugged into UV0, but TexCoord doesn’t do any better either.

Anyone got any ideas what might be wrong?