My girlfriend uses her iPhone to wake her up, and during night, she puts it in flightmode (because she doesn't like radiation).
1 out of approx. 3 mornings the phone doesn't wake her, instead it asks her (silently) for her Apple ID. The screen shows (in Dutch, so probably not 100% right translated)
after clicking at it, it tells
On your iPhone flight mode is active. Turn off flight mode to continue activation.
and after clicking this it asks for her Apple ID. After filling it in, just than the alarm clock goes off (off course to late :(
Strange behavior, especially since I have used this iPhone for a year (it used to be mine), without any of these problems.
Perhaps beneficial information: she has her own Apple ID, but for apps we share an Apple ID.
[iOs 7.04 on iPhone 4s 32GB]