My friend's was over 10k once.
Edit: He was being charged by the KB (or something) for internet access, when he was supposed to have a plan that gave unlimited internet access for $10 or so a month. It was his parents using the internet on the phones to surf ebay that jacked up the price.
Edit 2: I just called him. The bill is now up to a little over 17k. HOWEVER, when this first happened, they did recalculate the bill and he only had to pay what he actually owes. It's just that they still haven't fixed the bill to reflect that, so every time he logs into his account, it says he still owes over 17k when he doesn't.