While you're in co-op mode, open a players profile and inspect a players character there's a graphicbug while hovering any item. The world starts to flicker and it seems that the ground disappers so that the backgroundimage is visible. I don't know how to explain it, so here's a screenshot: https://dl.dropbox.com/u/66925220/diablo3/Screenshot019_2.jpg (hopefully it's allowed ^^)
The screenshot was taken in Caldeum.