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AND..... it's has happened to my 5s a few times after installing that hateful iOS 9. Really wish I didn't...... this problem and lagging with opening apps and whenever i'm in Settings. My 5s was working well with iOS 8...... lesson learned.
 
Have iPhone 6, 5s, 5c in my household all running iOS 9.0.1 - no problems spotted so far...
 
Probably restored from backup. Leave your old junk from that 3GS behind.

Like I said in another thread tonight, setting up your new iPhone as a new iPhone is probably the best way to go to reduce your chances of having issues. It can be a PITA, but so can a phone which turns off randomly.
 
I had a similar problem with the 3GS on iOS 5 a few years back rebooting during calls which was fixed with iOS 6. Hope people don't have to wait that long for this!
 
This happened to me once, with my Apple Watch, about an hour or so after upgrading to WatchOS 2. Given the close relationship between WatchOS and iOS, this very much seems like software issue (with iOS9 in particular).
 
And this is why I generally hold off buying any new gadget for at least 3 months after release.
 
This is a natural feature of every smartphone. If you're doing something extremely taxxing on the processor, it may shut off because of overheating. Perfectly normal.

Are you serious? A natural feature of a smartphone. Don't defend this crap. You're smartphone shouldn't restart its self. That is a sign it is something wrong with either the phone or software.
 
This would worry me (well, my wife) since we use our phones as alarm clocks. She hasn't seen this issue but her touch screen completely stopped working once. Had to hold down power and home to restart.
 
My iPhone 6S has never shut itself down - but I have had the odd moment where the phone has stopped responding. A quick tap on the home button has sorted it thus far. Can somebody direct me to the log file held on the device - next time it happens i will take a look if there is some form of kernel panic or something and post on this page.
 
I have an iPhone 6S Plus 16GB so far no issues with iOS 9.0.2 setup as new

Also iPad Air 2 iOS 9.0.2 setup as new never have any issues as yet.

From what people have been saying sound most likely some form of software bug.
Hopefully it gets sorted by apple asap
 
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