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?
