For anyone else experiencing a glitch like this, try turning off any blob shadow projectors you may have in your scene. Had me infuriated for days before I found it.

Actually, after further investigation, the problem is a side effect of having more lights than your Quality setting of "Pixel Light Count". While the result might be what you expect from a rendering tech standpoint, it is horrible in actual use when used with terrain. It only affects forward rendered objects, like terrain in deferred.

To repro, just take a piece of terrain, set Pixel Light Count to 1 and add 3 or 4 lights and slide them around the terrain. The terrain snaps to certain lights and results in nasty sharp edges on the lighting in the terrain.

This is a huge problem for us. The above bug doesn't mention terrain but this happens to us only with terrain.