I'm surprised it took 2 hours to sort it out. Well, not that it took that long, but I should think you would be passed up the call chain pretty quickly when the first person checked your account saw such an obvious error present.

The tricky part is working out how to cause the error to be in your favour. Computers generally used signed numbers, where once a number gets large enough instead of being a very large positive number, it becomes a very large negative number. It would be nice to have a debit balance of 23 quadrillion on your credit card :)

It is a little worrying that they don't have an automatic check at something like $10M though, where any larger transactions are obviously erroneous or need special permissions. Hell, any transaction that is 10x the cardholders credit limit would be a reasonable upper ceiling to automatically reject, too.