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the power buttons on 3 iPhone 5's that I have handled at length have all had some play (could translate into a rattle if you violently shake the phone). I would deem this "within spec" but then again we're discussing a 2+ month old thread. I'm sure the OP has either exchanged it or decided to live with it by now.
 
Obviously it's annoying that this has happened to so many phones when you pay so much.

I completely fixed mine in about 1 min, although some may consider it a dirty fix.

I cut a strip of paper from a post-it note, just slightly bigger than the power button (so yes,
Very very small) and put in in my case between the plastic and the phone.

First time I did it was a little to big as Folded a piece in half. It would not respond as it was completely depressed.

Anyway, try it if you want. Was driving me crazy but I am sane once again.
 
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