: You can't overlap vertices (it's not vertexes, it's vertices :P). That is
: most likely the cause of the problem.
Uhm, actually it's a little more complex than that.
Remember it's totally and completely impossible to have a solid with genuine empty areas above and below it... the various ways of faking the effect are all pretty poor...
You probably laid down the vertices incorrectly (ie creating them where they are, rather than moving that area so they don't overlap THEN creating the vertices then moving them around) or some such thing - I've overlapped vertices a bazillion times with no trouble as long as they weren't CREATED at the same place.
