How to build up a 1000+ € phone bill..,

... without you knowing.
And no roaming.
Or doing anything different ... compared to the previous month.

This is what just happened to a friend of mine:
She learned from their operator that she ran up a 900€ data bill, and the month (i.e. billing cycle) was not even over.
Did she have an app running, that kept data open? No.
Did she have a video running in the background? No. [1]
But she did have a 15MB email in her outbox that could not be sent for days.

The email app (on the iPhone) was trying every 3 minutes.
Could be tracked down with the operator's help.

My guess: it failed, because the mail server (or mta) did not let it send a 15MB+ email...
So it kept retrying, without any chance to succeed.

Simple math:
3 minutes a day = 20 times an hour = 480 times a day

Lets just say it successfully transmits only 1MB before the error[2], that's still 480MB ~~ 1/2GB a day.
Continue for a week or so, and you have 3.5GB... and through the 3GB ceiling and you hit the area where it gets really expensive.

And she really did not do anything wrong. Claimed that there was no error message, from the mail-app... And why should there, it kept retrying anway...

Well, the operator in an act of humanity - or to avoid the negative publicity of a court case - found a way to help here. But only because it totally breaks her data pattern until then.

--
[1] The operator really asked this... no pun.
[2] unlikely, it probably occurs after a lot higher transfer volumn, so those are best case numbers.