I was once training an intern on the concept of “negative testing”, and as an example someone shouldn’t be able to withdraw a negative amount of money from an account.
Upon a demonstration, at a multi-billion dollar financial company, it caused an error message but the account balance was actually incremented.
The quality of enterprise code is quite poor.
Not simply a clerical error , an internal process review and quality control error. No one person’s mistake should ever affect a persons account in an ideal financial system.
I once had a local credit union teller accidentally place my cash deposit into another person's account. An hour after deposit, I didn't see my checking account reflect the cash addition, so I checked the deposit receipt and the account number wasn't mine.
When I returned, this teller had already corrected her mistake (but I entered fuming/livid). She is now the branch manager and has been extremely helpful/attentive ever since.
Somebody messed up some COBOL script.
I once went to a drive-up ATM and withdrew $100. It deducted $100 from my account, but the machine gave me $300 instead. I walked into the branch and returned the money. They were thankful, but I never followed up with what happened. It was one of those small town credit unions. I guess weird things must happen frequently.