The secret of the $23 quadrillion VISA debit errors looks like a specific and not uncommon programming error. Take the insanely large number, if you convert 2314885530818450000 to hexadecimal, you end up with 20 20 20 20 20 20 12 50. In programming, hex20 is a space. Where a binary zero should have been, there were spaces instead. What made this instance special is that it wasn’t caught in time. A Slashdot commenter identifying himself as working in the industry explains more about what very likely happened:
I hope he cleaned his plate. Jon Seale was another of several VISA customers who were charged $23 quadrillion for mundane purchases. This time it was his July 13th meal a Dallas restaurant, reports KXAS. VISA said a temporary programming error affecting prepaid accounts was responsible for the error . Jon spent the rest of the day calling between Wachovia and VISA to try to clear the $23,148,855,308,184,500 charge.