To avoid this (common) issue, EU regulations now mandate a roaming-fee cutoff at €50 until the account-holder positively indicates their knowledge and acceptance of additional roaming fees. They can also pre-authorize a higher cutoff if they really do expect to be incurring more than €50 in roaming fees and don't want to be cut off, but this must be an explicit opt-in (not the default terms of a data plan). The result is that your losses in the case of unexpected data syncing are at least limited by default. But only within the EU, of course.
That was a big portion of the basis of my appeal; a carrier shouldn't allow charges to be compounded that high and that fast. There were alert SMS messages stating my data charges balance, but the $50, $100, and $200 alerts all arrived at the exact same time, literally all had the same timestamp. If I had been capped at $50 and then prompted in order to continue, I would have paid the fee and learned my lesson.
