From BlenderWiki
[edit] Concave quads
Sometimes when creating a face you may get an error message like this:
Specifically, that means there's a concave section in the quad you're trying to make. Here's an example of a concave quad. You'll get an error trying to make a face with this. See the middle vertex on the left? It makes a "dent" in the quad.
To fix it, move that vertex out a little bit. Now you can make a quad out of these vertices.
![[]](/skins/blender/open.png)
