I feel like there should be secondary checks for things like this. When I was running a MMORPG, I was terrified of duplication bugs (dupes). In many ways, dupes have the same effect on a game economy as this type of theft has on the real economy: it can go unnoticed for a long time, and the victims are primarily the masses (money supply) until someone finds out.

Since we didn't have any network security professionals on our team, I was especially worried. What we realized though, was that we kept a detailed log of all item/money creations/deletions, where trades were just a creation/deletion pair. Thus, we wrote a script to learn what the most expensive items (and thus most costly, if duplicated) were at any given time, and match creations to deletions with a frequency increasing with item value. Whenever there was a discrepancy, we were alerted.

I suppose banks could do something similar. If they separate the money transfer system from the account creation system, they could add an additional layer of security. I haven't really thought out the details, but it makes sense at first glance. Perhaps they already do something like this?