I'm trying to think of an Apple IPS display that hasn't had this issue to some extent. The iPhone4. The 27-inch iMac. The 24-inch LED display. My 2005-era 23" HD Cinema Display.
This is a manufacturing issue in which the multiple laminates which make up the display weren't perfectly sealed together. So it's possible for the display to pass initial quality control checks, but the reality is they're in the process of separating a bit. Odds are it'll slowly continue to separate over time, making the spot grow larger.
Apple has been hit or miss over replacing devices over the issue. On my Cinema Display - which was $1800 at the time - I received multiple (refurbished) replacements at the time, all of which exhibited the same issue. Eventually they gave up. Now the spot is about 2 square inches in size after 5 1/2 years. However there have been cases where replacement has been flatly denied and the customer was told there wasn't a problem.
The issue with iPhone 4's "yellow spots" may have a simple explanation. Users at Apple's forums have reported that the issue just "went away". Perhaps this post at the Apple Insider discussion forum sheds light on what's happening:
-- quote --
by user: austingaijin
The yellow blotches will disappear
Apple is using a bonding agent called Organofunctional Silane Z-6011 to bond the layers of glass. Apparently, Apple (or more likely Foxconn) is shipping these products so quickly that the evaporation process is not complete. However, after one or two days of use, especially with the screen on, will complete the evaporation process and the yellow "blotches" will disappear.
How do I know? I was involved in pitching Z-6011 to Apple.
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