I rented the game for the weekend, and some of the textures are...just fucking strange. A lot of the textures are rainbowed colored until I get really close to them. And right now, the road I'm standing on is made up of what seems to be a layout of a... character model or something? Hell, I have no idea. Is anyone else experiencing this? Are there any possible fixes?

I can't say I've ever had that problem. Maybe it's a defect with the disc? Try installing it to the HD if you haven't already, or see if the disc is scratched or something.

Nope, never experienced it ever on either the 360 or the pc

Sounds like something isn't loading correctly. I would hope it's just the disc that's all messed up and not your 360's gpu that's about to take a dump. 
 
It doesn't do this for any other games right? Try popping it into a friend's 360 and see if it does the same thing there. If anything you can do some split screen. That way everyone wins.

" As it so happens, my old 360 E74'd whilst playing L4D, with exactly the same symptons you describe. "

Doesn't happen to me, then again I play it on steam.  But I did play it for 360 but don't remember anything like that happening. 

I request photographic evidence of this, or I cannot conclude that this event you so graciously described occurred as you said it did. 
 
pics or it didn't happen

It has happened. 
 
Say goodbye to your 360 while you still can.

If I had to guess, I'd say that your GPU is acting up and is likely to fail. I hope I'm wrong.

" If I had to guess, I'd say that your GPU is acting up and is likely to fail. I hope I'm wrong. "