Experienced it first a few days ago, only happens occurs in certain angles...
EDIT: And only at certain materials/textures afaik
Anyone know why and/or any way to fix it?

Don't look at those materials at those angles.

Sorry I can't be of any more help than that, I haven't used Hammer for a couple of years.

I used to get this when the camera view for 3d was set to "3d shaded textured polygons". Setting it to "3d textured" solved the issue.

groxkiller585 wrote:
I used to get this when the camera view for 3d was set to "3d shaded textured polygons". Setting it to "3d textured" solved the issue.

Yeah, that view mode is very glitchy in portal 2 versions of hammer.

I get it in regular SDK whenever I place an npc_citizen. Doesn't affect the compiled map though.

groxkiller585 wrote:
Setting it to "3d textured" solved the issue.

Solved.