After applying a GUI revamp resource pack, there are two red pixels. And I think I found the reason. I opened anvil.png in Photoshop, selected "Quick Select Tool" and I found out, that the spot where the red pixels are located is transparent (because you can't select it by QS tool - see photo 3).
I doubt that it's resource pack maker's mistake, because the "big red area" is unchanged if you compare it to default.
Sorry for my bad English.