Given the weather we have had recently if you see the battery indicator going backwards it might be temperature related.

I was out in very cold weather (~ -15C) and the phone turned itself off, just as I was needing to make a rescue phone call.
I stuck the phone under my armpit for a while for heat and managed to turn it on and coax a phone call out of it. On returning the phone to room temperature the battery was back up to at least a quarter.