Texture Problem
I noticed 2-3 walls in my latest level have an odd diagonal slash through them, they are just ordinary solid brushes and there is nothing unusual about the way the texture is aligned in hammer, any help would be greatly appreciated.
It sort of looks like an issue with reflections. It's possible that the engine is splitting the rectangular surface into two triangles and then using a different cubemap for each triangle? I can't say I've ever seen this before though. If you have a cubemap close by, you could try assigning it to that surface and see if it changes anything.
Portal 1: Portal Pro, Resurrection, Dyad, Rexaura
Portal 2: [Coop] Electrophobia, [PTI] EotM Collection, [SP] Aperture Halloween
Thanks Mevious, I tried but it didnt work so I didnt compile the cubemaps at all and its still there. It affects 5 brush faces all of which are in the same room. Its weird the cut is always that same angle because one of the brushes is 256 units across but the slash only covers 128 units on the left so it isnt slicing the entire brush.
EDIT Just put a func_viscluster in the room and that didnt work either, although I didnt see any unusually shaped visleafs anyway.
Could it be a vertex texture? Those are meant to be rendered on triangles.
Well, I've had the same texture issue a lot of times 
Overall in Da-Mask, and it was even worse because the textures I used were plain and those triangles were more noticeable.
Due to the places I found that bad lighting in, I suspected they were caused by shadows casted during VRAD compile. For example:
I corrected it (in a TON of places in this map) by turning the brush into a func_brush setting "Disable Receiving Shadows" to YES.
I hope that helps! (do not get too mad at that kind of manic stuff because you could end up crazy! like me )
useful tools and stuff here on TWP 
[spoiler]
[/spoiler]No its not a vertex texture.
Thanks Josepezdj, suppose it doesnt matter too much, it is set in the destroyed theme so it kinda makes it look the tile is cracked but yea best not going too crazy trying to fix every little detail 
I will try changing the lighting and func_brushing though.
EDIT: Func_brush + Disable receiving shadows worked, thanks Josepezdj 
I'm glad to hear, my friend
useful tools and stuff here on TWP [spoiler]
[/spoiler]
I always wondered what caused that. I thought it might have been a cubemap issue.
I'm curious now. Does it still happen if you assign a face to a cubemap?
I'm curious now. Does it still happen if you assign a face to a cubemap?
Nah I tried that, and if you don't build cubemaps at all its still there, assume its just a weird shadow problem then cause func_brushing, disable receiving shadows fixed it perfectly
It might still be related to cubemaps, even if you don't insert or build them.
A lot of Portal 2 materials have their own standard cubemap, which is always the same in each map. (Kind of annoying, since it doesn't adapt to the environment, but that's a whole 'nother issue.)
I have exactly that same crack in one of my maps and it just refuses to go away. Incidentally, it's a wall above a button base, just like you have there. I wonder if the brushes in the button base have something to do with it?
Other Portal 2 maps: Medusa Glare
Portal 1 maps: Try Anything Twice | Manic Mechanic