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Apple may think it's a chemical reaction. When I called, they did ask me specifically what body lotions I used.
 
This has happened once to me. I've had my series 0 since day one and this happened in August( 15 months since I received it). Luckily, I had Apple care and did the express replacement. It came off when I picked it up off the charger and leaving it overnight. I use the charging card that came with it.
 
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