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I've observed this problem after updating iPhone 5 to iOS 8: single click the home button would sporadically bring up the multitasking screen, instead of going back to the home screen. This appears to happen regardless of what app I single click the home button in.

I have since performed the following:

1) Settings / General / Accessibility / Home-click Speed both at Slow and Slowest
2) Settings / General / Reset / Reset All Settings
3) Restore using iTunes, set up as a new phone

The issue persists.

I have then downgraded back to 7.1.2. The issue no longer appears. So this does not appear to be a hardware issue.

Anyone else seeing this?
 
It happened to me too after the iOS 8 update; I'm on an iPhone 4S and I'm quite pissed off. Any update?
 
I'm in touch with Apple Support to diagnose the root cause of this issue. Meanwhile, I've created this thread on Apple Support Communities so affected users can report their observation:

https://discussions.apple.com/message/26618981#26618981

I will update this as I receive more information from Apple.

-alex

It happened to me too after the iOS 8 update; I'm on an iPhone 4S and I'm quite pissed off. Any update?
 
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