All it took to trigger the widespread woe was an outwardly insignificant slip: on September 17th, Nets Estonia coordinating card transactions in Estonia forwarded a file with cards transactions to the financial institutions twice, and two days later attempted to correct the mistake by sending a file cancelling the «double» transactions.
The banks which for whatever reason only acted on the cancel-entries sent on September 19th, yesterday morning unexpectedly returned to customers the money spent on September 17th. This, for instance, was the lot of SEB clients. To our knowledge, clients of institutions like Swedbank and Citadele were less lucky. The control systems of said banks had already acted on the double file dating September 17th and brazenly pocketed the customers’ money twice.
As LHV and Nordea banks control systems pulled brakes both on the file prescribing double payments and dataset sent to cancel it, the clients of both escaped the mess.
Why LHV and Nordea engineers could implement fault tolerant algorithm while engineers of the two biggest banks SEB and Swedbank could not?