A user managed to hit O2 1GB data usage limit on iPhone 5 in two days! He was normally using under 100MB per month and could swear that he hadn't done anything unusual in these 2 days.
After checking that backups and updates over cellular were disabled and there were no suspicious apps running, I added additional data package and reset iPhone data counter. In couple of hours I checked the iPhone again and to my surprise it had already uploaded over 40MB of data over cellular network.
Before resorting to resetting the phone to factory defaults, I opened the email app and noticed an email stuck in the outbox that iPhone was trying to send. This was a MS Exchange account. An email was a large one with multiple images attached. It seems that the email was exceeding the size limit set by Exchange administrators, but iPhone was trying to send this email again and again, running through mobile data caps in no time.