It looks to me like you're using some kind of antialiasing that doesn't support transparency.
If you didn't implement the AA yourself, maybe the graphics driver is doing it. NVIDIA Control Panel has an option to enable "Antialiasing - FXAA", and there's also an option called "Antialiasing - Transparency", in the "Manage 3D Settings" option. So either disabling the "Antialiasing - FXAA", or enabling the "Antialiasing - Transparency" will get rid of that white line (if it really is AA).
Not using any AA. I checked the driver settings and they're all set to application controlled. Apart from 'Gamma Correction' but it says that only affects GL.
It looks like that part of the terrain is not in the lights volume frustum.
Outside of the point light in that screenshot the rest is lit by a full-screen quad directional light.
I honestly have no idea what's causing it, based only on what you've said here, but the first couple of things that spring to mind when seeing that kind of artifact are checking to see if the edge is blending with something it shouldn't (AA artifact) or possibly there's z-fighting, since that's at the far edge. Those are just stabs in the dark, though.
Could you give some more detailed information about the process you're using?
The correct forum for this kind of question is 'graphics programming'. (It's not a big deal, though.)
It's not z-fighting. I'm not sure about the blending. The 'process' is pretty hacky as I'm still learning. Not sure what information you need but I'll try and elaborate.
I'm rendering position and normals in to an MRT. From that I'm rendering the lights to a render target. I then take the lighting render target and project it on to the geometry as a sort of, I dunno, screen space lightmap. I'm pretty sure it's the projective co-ordinates that are wrong. I've never understood the texel offset stuff in D3D9.