It would appear that the traditional banks have not yet solved the Double-Spend problem (as in double-spend your money even if that means you get a negative balance).

Perhaps they should switch to blockchain technology?

https://www.aftonbladet.se/nyheter/article19660429.ab
http://www.svd.se/naringsliv/visa-kunder-fick-betala-dubbelt_3989501.svd

One of the biggest Swedish banks, Nordea, decided that spending customers money (charging them) twice each time they purchase something with VISA was a great idea. It appears that they did this for a full two days.

This only affected 2 million VISA card holders or 1/4th of the Swedish population. The one-fourth of Sweden who now find themselves with negative bank account balances are not amused. Some have pointed out that not having the money needed to buy food and pay bills available to you is not that fun.

Nordea has promised to "sort this out". Nordea will surely deliver on that promise. We just have to wait.