As the crisis management consultant for trading companies, I can say what the impact will be -- about two weeks of sleepless nights and endless phone calls for the likes of me, grabbing all the data they can find from old backups, other counterparties, clearing agencies, and other guys and girls from the banking tech mafia, and plugging it back to the databases while the executives make their best poker faces and try to convince everybody that everything is all right.

Stuff like that (maybe with less scale, but still quite a lot of damage) happens occasionally... by accident, and thankfully stays unknown for the general public.