Doesn't matter how good you think are. If you write code professionally for long enough, you eventually experience an event like this first hand. It's an absolutely horrible experience for a developer.
So stop making excuses. An organization of this scale should be taking serious precautions (against code errors, bad hardware, malicious attacks, network problems, compiler bugs, cosmic ray induced bit errors, etc.) when building a credit card processor.
If something truly extraordinary comes out to justify this problem, I may be sympathetic. But Occams razor suggests this was avoidable (through good process, not "I should have seen that off by one error").