Browsers: IE 7, Firefox 2, Opera 9
Example: Look closely at this site, you may have noticed the effect already.
Grab the left or right edge of your browser and alternately shrink and expand the window past 920 pixels.
As you expand the window past 920 pixels, a slightly transparent image appears in the top left portion of the screen, yet it is NOT seen behind the main content portion where you are reading this now.
I stumbled across this effect purely by chance while designing this site.
Complete write-up to follow.