I had one weird bug where, I restored my iPhone 5 (using the Restore All option from the Settings app) and the next day my alarm goes off, as it used to be set to before the restore.
Then it hit me -- HOW?
I opened the Clock app and sure enough -- NO ALARMS were present -- as it should be when a "full" restore from the Settings app is performed.
Scary, scary, iPhone ghost? But still -- it woke me up (and I even had to swipe it silent) that morning... which was a good thing.... friggin' freaky though.