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Occasionally my macbook pro (15-inch, Early 2011 with mountain lion 10.8.2) displays random garbled blocks. This happens on external Apple displays and the laptop display. See photo.

enter image description here

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Are you using gfxCardStatus by Cody Krieger? (If yes, update to the latest version.) – MacLemon Sep 29 '12 at 19:12
This problem continued to persist. I had to get the logic board replaced to fix this issue – tommy chheng Jan 23 '14 at 20:36

We had a lot of these problems in our store and in the most cases the graphic card was broken. You should run the Apple Hardware Test / Apple Diagnostics and look at the results.

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I've had the same thing on two machines. The graphics card was bad and Apple replaced it under warranty. In one case, twice in about 5 weeks. – Mark Jan 21 '14 at 17:13

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