I use a MacBook with a secondary monitor. When my screensaver initiates, there is a static black rectangle obscuring a significant portion of the image on one of the monitors. Sometimes it's on the MacBook monitor, sometimes on the other one.
When I turn mirroring on, the problem goes away, but I must have mirroring off to work.
When I turn mirroring on, the problem goes away, but I must have mirroring off to work.