rectangular artifacts during netflix streaming

I've recently begun seeing random, darkish rectangles appear and disappear while watching a streamed Netflix movie. Last night when it began happening, I disconnected my laptop and substituted a friend's, whose is almost identical to mine: hers is early 2011, mine is late 2011. Hers showed no rectangles.

Several facts:

-- viewing in fullscreen mode

-- using video mirroring to display on TV

-- using WiFi

-- latest OS and Silverlight software being used

-- no other apps open

-- both laptops are MBP 13"

Can anyone speculate as to what these randomly sized rectangles might be? Are they artifacts of video compression or streaming? Could they in any way indicate a failing video card?