A fire extinguishing test at a Bucharest data centre belonging to the Romanian arm of ING saw the bank's customers unable to make card payments, ATM withdrawals and online transactions for several hours on Saturday.
The bank says that services were knocked out for around 10 hours thanks to the damage caused when it tested its extinguisher system, which expels a mix of nitrogen, argon and carbon dioxide, known as inergen.
Inert gas is used to combat fires at data centres because it does not normally damage electronic equipment. It is stored in cylinders and released at high pressure out of nozzles spread around the site.
However, according to Motherboard, on this occasion, the high-pressure release of the gas produced a loud noise - over 130 decibels - and the associated vibration destroyed dozens of hard drives.
In a statement, ING Romania's head of retail banking, Daniel Llano, apologises for the outage. He also says that customers were not updated sooner because the problem also hit the bank's ability to communicate via text and email.
Llano says that "diagnostic analysis" is being completed and, along with its suppliers, the bank is monitoring the fallout from what he calls an "unprecedented" incident.