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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.
 
Defective. If the volume buttons are loose it's also a defect, again, take it back!
 
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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