I've seen this too, in my case it was caused by a combination of N-gons (faces with more than 4 vertices) and the SreeD preview renderer (the alternative to OpenGL preview if you right-click in the poser window). 

I don't usually use Poser's SreeD renderer/display, I was checking it out to see if it helped speed up my OpenGL display in Modo which was running concurrently. I got lots of polys attaching to the top corner of the screen like that, all from n-gons. They don't render, but they look awful in the preview.