So far, the Landscape Editor is my favorite customizaton option, and I had never had any problems with it... until now. I've added pictures so my vague description doesn't confuse you.

So today I decided to make a slope level:
http://img216.imageshack.us/img216/5773/fullvieweditorpw5.png
Simple, but fun, I thought. Notice how it covers the whole level.

I then clicked on"Custom Game" and loaded my landscape.

Then I scrolled left:
http://img490.imageshack.us/img490/1774/whatthebi1.png
OK This is NOT how I planned it...

Scrolled right:
http://img223.imageshack.us/img223/3115/whatthe2ok5.png
There's definately something wrong here...

In the landscape editor, there's a red area indicating that you can't draw there. My slope has its limits in both red areas. But they show up with no limits when I load them.

Which leads to this:
http://img296.imageshack.us/img296/9216/fallingstartslope2it7.png

What can I do to fix this? Is there an option I missed?
Thanks in advance. :)

P.S. Sorry for image quality. I took the pics with my phone.

Easy mistake to make; I've done it a good three times.

You want to put a border on your level. Open the tab at the top, and from the root menu, choose the fourth icon from the left (Landscape Settings). Then from that menu, choose the first icon to turn a border on. Unfortunately, if you do this, airstrike, donkey, bunker buster, etc. will no longer be available :(

Thanks! It worked.
Too bad you can't use air based weapons :(