How do i fix this?

How can i fix these lines in my room? Currently i have no lights in there but i can see lines outside this room too where there are lights.

Built cubemaps? Checked that it’s all sealed?

Yes.

Cubemaps can cause crappy reflections if not used correctly, just remember this, the materials sample reflections from the nearest cubemap, which means that if you have one in front of a light, and no one where it’s dark, the materials in the dark will sample reflection from the light area, thus making light reflections in a dark area.

Lines like this can also be down to way the source engine handles Anti-Aliasing.

Which in that case, they cant really be fixed.

What game did you take this picture in? This only happens to me in Garry’sMod.

Yeah it was Garry’s Mod.