Anyone know what causes this? (Photoshop)

Often when I open an image in Photoshop CS6 parts of the image don't appear ...

In this example the top left and top right of the image is missing; other times it may be elsewhere; often it's just the top left.

It's no big deal as I simply do something to the image (eg Auto Color) and undo it and the image is fine. But I wondered whether anyone else has experienced it or, better still, knows the reason.
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