My wife has literally a penny in her account at any given time, so there is no chance of data overage. Haven't seen any similar issue though. Only problem I had was a few months ago I went to renew her account, and when I did for some reason it created a whole new account over top of her old account, and her phone was still attached to the old one. There was no way for me to see this in the system, but when I called in the CS rep found it and was equally confused as to how it happened (both "accounts" were tied to the same "account", but the phone wasn't). They fixed it up for me though without too much problem, and after about 30 minutes all was well again.
Needless to say, their billing system isn't exactly bulletproof.