My mom works in customer service for the phone company, she has plenty of stories about customers calling with such high bills. You are paying per MB. If you do not realize how much it's costing you and start downloading lot of apps off data instead of using wifi, or you do use wifi but it kicks out and you don't notice, it accumulates fast. They do give 1 chance at waving a bill though. My mom actually got a customer that had a $150,000 bill which was waved. He was just downloading lot of apps, doing lot of internet surfing on this phone, and just never realized how expensive it gets when you go over the cap, which is ridiculously low. Really with smartphones being fairly standard now, I don't get why data plans are so expensive and so capped.
Even as a smart user, the thought of that being possible actually kind of scares me. Let's say I'm out of town or something and just decide to check facebook from bed, and fall asleep, and it stays on (auto refresh). That's going to be one hell of a nasty bill for such a easy but stupid mistake. Even worse once you get into roaming territory.