Where pixel segments of similar length are stacked, they appear as big clumsy clusters ('fat pixels' as Helm calls them, check his 'rumblethread' tuto at Pixelation if you haven't). Once you've seen them, you can't unsee them ;)
Fixing is usually a single pixel matter.
Top left corner of the rock, below the tree: double green segments; below that: stacks 2 px wide, up to 5 px high: these are the most obvious.
Edit: Ramblethread! LOL @ rumble XD
Hmm, I'm not sure where exactly the banding is happening. The lines that follow the contour of the rocks and stuff were just a motif I was playing around with. Is there something I'm missing?
Ah! Okay, I understand. Thanks for the help! 'u'