I did code-level support for a large internal data management system for the federal reserve bank in the mid aughts. I'm not really interested in disclosing the specific bland but critically important data we moved/managed. It involved a giant amalgam of legacy commercial AIX and HP-UX software with some newer Java stuff in a Websphere environment. It was all duct-tape-and-bubble-gummed together with huge ksh88 scripts that were in constant development by devs and support people like me who'd just ssh into production systems to make changes. None of the internal software had dev/test systems, there was definitely no code review, and there was no version control. Yep. The system on a whole required constant monitoring and intervention to keep it plodding along. We did periodically see data loss that was unrecoverable from a technological standpoint but recovered with physical media kept around from their necessarily belt-and-suspenders business practices and tons of available staff for data entry. I reckon tens-of-thousands of folks outside the bank would have been affected per incident.
But compared to, say, tens of thousands of investment transactions for pension and retirement funds evaporating, disbursements to a hospital's operations budget being delayed, sending confidential historical balance records in bulk to a large corporate customer's competitor, having an online investment system down during a period of extreme market volatility, or even large-scale customer data breaches, then yes. 'Money missing from a bank account' is in every significant way a comparatively minor problem probably fixable with a phone call and a couple hours of investigation on the bank's part, even with a significant sum of money. The bank would almost certainly catch it themselves during an automated audit. It's exactly the sort of problem that bookkeeping organizations are designed to mitigate and root out if they do happen.
The others could affect many, many more people's lives for much longer— potentially permanently.
- took a trip to the neighboring country, my friend tried to take out money from an ATM; first two transactions failed, third succeeded (at the atm). In the bank though, all "succeeded", and money were gone (first 2 eventually were reversed with manual intervention).
- I sold a house (in a remote mountain village), buyer sent 8100EUR from Germany, 78xy.z EUR made it to my personal bank account. Apparently due to multiple currency exchanges, but this is a SEPA transfer between two accounts both denominated in EUR, this was absolutely not supposed to happen and nobody was able to articulate exactly "why it happened" (or even exactly what happened). For this one the buyer decided to just eat the loss and sent me an additional 300EUR.
* https://www.theguardian.com/money/2015/aug/28/many-hsbc-cust... (I believe this one was caused by firing greybeards who understood a certain very complicated batch processing system and offshoring to undertrained staff in India who then make a mistake and basically brough the entire system down as a knock-on effect)
* https://www.theguardian.com/business/2019/nov/19/tsb-it-melt... (Insufficiently tested software upgrade although to be fair this was a major upgrade)
* https://scottishfinancialnews.com/article/rbs-it-failure-pre... (Not sure on root cause. Other interesting stats at the bottom of the article regarding failure counts)
Now, in all these cases I believe the outages lasted a few days at least where people had limited or no access to bank accounts, salaries not paid on time, incorrectly charged overdraft fees, automated payments not happening etc.
It can probably be argued what is "core" here and as far as I know no money actually disappeared into the ether but, in the UK at least, I am not sure I would be so confident in my declaration of Rock Solid software.
If the software isn't extendable or changeable then it isn't 'working fine', because if you can't offer internet banking or do FATCA reporting you go out of business or get shut down.