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I'm pretty sure they do come with some kind of warranty. Just go to the Apple store, they'll probably take your old one and hand you a new one. I've read several people have successfully done this.
 
It's covered but you have to get same color. I had to return mine, the blue one because the color changed. They made me get the same one. so a week later the color changed again. I dont bother anymore with the Apple bumper. I buy the knock offs on ebay for so much cheaper
 
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