Bug in extrude? ( Possibly just a display bug ).

 From:  Crusoe the Painter (CRUSOE)
So to learn Moi, I decided to design a 'blast rifle'. I made the outline from various intersecting lines, arcs, and splines, trimming and joining as I go. Then, I took the final stock outline, and tried 'extruding' it, and one part of the rifle stock comes out looking 'twisted'. The surface in that section also doesn't look curved, but 'flat' when rotated through some angles. I tried recreating the problem using a simpler outline, but have so far been unsuccessful.

Hmm, when I export into obj, and look at it in the subdivision preview, the generated polys look fine.