Mystery rendering error

Ok. I started a CTF map called Castle Lake Zone.
I make a basic thok barrier and water. I start getting a rendering error appearing in the sky while the outer edge of the thok barrier's visable.
I've checked and re-checked the thok barrier, and everything seems ok. Except that it's causing the rendering error, of course.
You will need Knuckles or Tails to see the error, as the thok barrier's set up pretty high.

Here are the facts:
A. It's in OGL only. Everything's A-Ok in Software.
2 = This has long since been happening recently with EVERY map I made from scratch recently. There are no problems when I update my older work.
Three: The water FOF's not the problem. I removed it, and all is still not well.

This error is bugging me. I can not find the problem, and I require assistance.
Check this map, tell me what's wrong, and mock me for missing the obvious solution.
http://www.sendspace.com/file/rm7kcm

From the sound of it, you're probably getting the Hall of Mirrors effect, am I wrong?

I'll check the file now.

Edit: I found that adding in a second sector fixed the problem.

That's bizarre, but thanks!

I found that out first. the same trick seems to work on sky-based redwalls, oddly enough(actually, that's how I found the trick).

JTE told me that you should make the Floor height the same height as the celing/Visa versa...
that should make the "HOM" effect go away

this is the exact problem i had in my Volcanic raceway map