Well, ive had the derpy lighting bug. and it seems entirely related to "some mod" with a transparent (self rendered block) not restoring openGL state properly when its done rendering its stuff, resulting in a cascade of shading problems for stuff subsequently rendered. As the render order changes (as you move the viewport around) this alters the point at which the derpy renderer stuffs with the openGL state, and changes - randomly - the subsequent shading.
Tracking down the mod responsible is no small task as, if you really wanted to pin it down, you would need to exhaustively add / remove one mod at a time, restoring the world each time as the block removal caused by removing a mod, will doubtless perturb the world enough that replicating the shading issue will change anyway. And I found the problem would only manifest after a while.
Unless of course, someone with sufficient expertise (I certainly don't) writes an OpenGL call validation mod that can track OpenGL state before and after each mod does its draw calls, to validate it hasn't been altered in a bad way.