Maximum distance for viewing textures?
Ah sorry, I instantly thought of a possible solution 
Try using linked_portal_doors. I've been messing around with this and figured out (as far as i know) that you can change the 'width' and 'height' to a cap of: W=256 - H=512 (this is x2 ingame).. More than that, crashes my client.
You would probably want to look up around for more info about this, rather than having me explaining, trust me ^^
I believe that's called the Hall Of Mirrors (HoM) effect, and it generally appears in 3D engine games when there's nothing to draw, so it draws the pixels immediately next to the open space.
As for the problem, you could have the consecutive portal placement go around a corner, or something of the sort, if it's really important that it be that long.
I'm kind of curious to see the VMF, honestly. My first guess was he was using a model texture and it was giving the standard "using a model texture on a brush face so you get weird VertexLitGeneric lighting" bug. And I don't think that mironos can explain what he is doing improperly in such a way that we can really help him.
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Thanks for the responses guys -- haven't been mapping for a couple of days, but now back.
I don't think the issue is texture-specific (though it's possible it's the WAY I'm applying textures -- this is my first map, so any sort of "user error" cause is possible 
). I've tried using different textures, and still get the same effect.
Additionally, I don't think my issue has anything to do w/ room geometry/portaling/doors, as I tried creating just a really long, empty room, w/ only a spawn point, and the same thing happened. I could try putting the spawn point in a different room, since Rderasta mentioned that as a potential cause. I'll give that a shot.
In the meantime, I'm happy to provide my .vmf if folks are curious. Sorry, I'm new to these forums, too...assume I can't just attach it to my post(?) So if anyone wants to take a look, let me know, and I'll find a place to host the file.
You can, in fact, just attach it to a post.
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Ah. So I can...
Ok, attached. You should see the issue when looking toward the metal door -- it's the darker space between the door up high and the ramp down low.
Ok, so noticed msleeper's post that the Portal2 editing tools were now available while checking this thread. Downloaded, installed, then reopened and recompiled my map...and voila, issue is gone!
Will probably make some changes to the map and compile again to verify, but for now it looks like this is solved! Thanks all!
Bizarre. Glad it's working right for you now, though.
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I would hazard a guess that it was some compiler optimization for the Alien Swarm tools, because Alien Swarm was a different game and they could cut different corners on their maps.
Yeah that kind of makes sense actually, but I thought it was the game client that did that and not the compiler.
Also Hober check your damn PMs or get on Steam.
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