Have you tried calling apple's hotline?
It may be time to return my applecare warranty extension since I just bought it and have yet to activate it on my machine. And one of the biggest reasons why I bought it was due to this thread. Even though the risk might be minute, the outcome and repair costs appear to be devastating.
But if they classify it as an aesthetic problem/user mishandling, there must be something wrong!
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Personally: I've used my rMBP for 11 months, have never babied it and the aluminium on the right hand rest is pitted with many micro pits (due to the perspiration from my palms), have been fortunate not to suffer from this problem, but I would doubt that it's metallic deposits or transfers/thinning of any ionic compounds induced from the handrest's corrosion.
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I'm quite curious if the origin of the LCD panel (since glass and matrix are fused) has any relation with this. Although the fusing process is downstream from LCD matrix manufacturing, Apple may just have more than one factory doing fusing for the two LCD matrix brands (LG/Samsung) separately.
Anyone else in this thread suffering from this problem could care to comment what brand your panel is?
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Anything to do with EU laws?