HaloMods "Classic" Read-only Archive

I'm trying to build a box around my map for the sky; so I created a plane above my map, and started creating faces to connect the plane above my map to my actual map. A lot of the faces get created just fine, but others pop up with the "Illegal new face" message.
I don't have much understanding as to what is an 'illegal face', except for a few instances. Could someone help me out and suggest a couple scenarios where this message would pop up?

if you create a face that is co-liniar or extremely non-planar.

Alright, so what about this? I don't really understand what I'm doing wrong on this one.



Edit:
I'm getting help.