See the following bug for a code sample that reproduces this issue:
Drag the dock widget from the left to the right. Notice the dock widget doesn't redraw properly and leaves a shadow on the screen where it once was. Keep re-sizing the main window or moving the dock widgets around and the problem gets worse.
Sometimes when this happens the "shadow" seems to "burn" into the screen as it will stay there forever even after the Qt application has been terminated. The only way to clear it is to log off and log back in!