Yep I agree with you Ajax..., but here the problematic is/was... not a concave poly... but a concave "angle" ;-), actually a planar poly with L shape, for some reason the rendering engine tends to "forget" some vertices. After testing... seems that everytime you have a concave angle, you need to join it with the other side of the poly, it looks like if the edge maintains everything in place, almost like a rope would. With convexes, you have no problem at all, if you make an hexagon and extrude it, it will render without any problem. Well... back to work now... :-) I have additional work paying attention to concave angles now... man I hate poser :-) See ya...

Vir sapit qui pauca loquitur.