Edge of sector not rendered properly

[742mph]

A particular sector inside a cave in a partially completed level of mine doesn't seem to connect properly with the outside of the cave. Instead of the flat bleeding out, though, the outside area below the floor seems to be bleeding in. I've tried flipping the linedef that connects the inside and outside, re-drawing the linedef, re-drawing the entire sector, checking for zero-length linedefs, etc. but nothing seems to work. And yes, each side of the linedef is part of its proper sector. Here's a screenshot of the issue: http://postimg.org/image/wj35ujnsh/

The level itself can also be found here: http://www.sendspace.com/file/5exqch

(No, my level is not going to be completely enemy- and ring-less once it's finished. Don't worry.)

 

[WellDoneSnake]

I think it happens because the main sector for that tower area is split up into sectors that carry/share the same sector properties (like when you join a sector to another indirectly), although I might be wrong. You could try rebuilding that tower area again (I call it a "tower area" because it looks like one).

 

[742mph]

I tried your method, and it fixed the problem in the tunnel, but moved it to a ledge outside the tower, and when I used it again, it became a slime trail on the edge of the ledge. That's when I tried repositioning the vertices of that area a bit so they were closer to the 32-fracunit grid, and voila, the issue disappeared altogether. Thanks for your help.

 

[WellDoneSnake]

No problem, I had this problem 3 months ago, I learned to solve it myself but I learned how to get rid of slime trials with the help of the members on this forum.
Your problem might have been that the the divided sector was illegal sector, but wiki doesn't show the illegal sector that you had so...