Im not sure if this is documented, but on Window 7 when a window is minimized and the desktop is switched and switched back the minimized windows lose their previews.
Also itunes loses the ability to hover over the superbar and use the controls from preview when switching windows whether its minimized or not during the switch.
Another issue is desktop preview doesnt recognize "snapped" windows. As in windows docked to the left or right win Windows 7. It only takes into consideration their last undocked position