I tried to write my own renderer and it does work fine so far, however, when rendering the south end the west face I happen to get a strange bug.

 

This is what it should look like:

And this is what it actually looks like (it's the opposite side of the same structure):

 

I'm using the ISimpleBlockRenderingHandler interface, and this is my render code (based on RenderBlock.renderBlockWithColorMultiplier):

Source on github

 

Does anyone have an Idea what's wrong there, and more important how to fix it?

OK. It seems that I have to set RenderBlocks.flipTexture to true during the problematic face renders. If anybody else gets this error: The problem methods are renderEastFace and renderSouthFace.