This sort of thing has been going on a long time, I'm sure. I think it used to happen to one of my own old accounts when I was a Maryland National Bank customer (my first account set up by my parents when I was a child, actually). When I was first working in high school and during college (late 80's, early 90's), I'd often go to deposit my meager minimum wage pay checks, check my balance and see thousands of dollars in my account balance that shouldn't have been there (or I'd see it before I deposited it). I'd go back to check a day later and the extra money would be gone and I'd have my proper balance in there again. I figured people were making money disappear in the system by moving it around and then erasing the electronic paper trail. I never lost money because of this, but I always found it odd and rather shocking.

It might have even been done automatically, with a secret worm program in the system. Maybe the worm keeps moving money around so it's invisible to the system, but it can also reconstitute all the money in hundreds or thousands of accounts when the fraudster wants access to it. They might have also been sucking a few pennies off of accounts, a few here, a few there, but do that from everyone's account and you suddenly are making a ton of money and most people won't notice a thing.

(MNB eventually got bought out by NationsBank, which then bought/became Bank of America.)