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"The symptoms of this problem first seem to after appear about 2-3 weeks of use of the Macbook. As you can see in the image to the left, most commonly, yellowish-redish spots appear on the plastic where the palm rests, and on the trackpad button. These spots cannot be removed by any cleaning solution, and seem to only get worse over time."
http://arstechnica.com/journals/apple.ars/2006/6/12/4297

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