GMail will not repaint fully if the window is slightly scrolled. I've found that it's easiest to reproduce if it is scrolled just enough to cover the top bar with the address and Settings, etc links. After it's scrolled, clicking to read a message will hit this (see screenshot).
On Vista, using the WebKit shipping with Safari 525.9 as well as the r30551 nightly. Have not tried on OS X.
Created attachment 19352 [details]