It will sound familiar. You’re doing an electronic transfer for US$10 and just stop short of sending US$1,000 by mistake. Sometimes bungled transactions like this go through — but rarely on the scale of last week’s US$900 million payments blunder by Citigroup Inc., which paid debt investors roughly 100 times more than it was supposed to.

The problem here is not just the size of the giant bank’s mistake. A history of bad will between Citi and some of the recipients is making it hard to undo the lapse.

If you gave someone an unintended windfall through an erroneous bank transfer, you’d have a reasonable expectation that they would recognize the mistake and it would be quickly and amicably undone. But simply asking for the money back isn’t working for Citi. So it has reached for the lawyers.

How Citi made the hapless payments remains unclear. According to its court filing, they were meant to be interest on loans issued by cosmetics group Revlon Inc., for which the U.S. bank says it was the “administrative agent.” Instead, it paid out amounts that add up to the full principal sum of the loan. This colossal excess came out of Citi’s own funds. It’s not as if Revlon, struggling with the pandemic, had suddenly decided to repay its debts early.

Citi says that there are going to be human and technological errors now and then. Sure, people make mistakes. “Accidents will happen” became a cliche because there’s an inevitability to such things. But nearly US$1 billion, in multiple transactions, from an institution of Citi’s sophistication? Fallibility and gremlins are why systems and procedures need to be robust. When those checks don’t work, that’s an institutional failing — especially at a bank capitalized at US$105 billion.

Citi now faces a battle to get all of the money back. Some US$175 million is attributable to a single hedge fund, Brigade Capital Management LP. Citi quotes an email from Brigade suggesting that, regardless of any errors made, it sees legal grounds to keep the payments in order to set them against its exposure to Revlon. On Tuesday, Brigade told a judge it doesn’t have the funds as they went to its end clients, Bloomberg News reported. The judge has granted Citi a freeze on the cash for now.

Meanwhile, a wider group of creditors represented by UMB Bank, part of UMB Financial Corp., is now suing Revlon and Citi, arguing the cosmetics group is in default due to the way it conducted debt restructurings earlier this year. UMB says the debt rejig wrongly stripped its clients’ loans of collateral. A default would provide grounds to demand immediate repayment of the principal owed.

Citi says this is just a pretext not to return funds erroneously transferred. Revlon has dismissed the suit as baseless.

The timing of this creditor lawsuit certainly looks fortuitous. It landed a day after Citi’s botched payment, yet it relates to debt deals that took place months ago. The creditors could have taken action sooner. It looks like they were ready: The filing is some 117 pages long. But it’s one decision to launch a claim to recover money you don’t have. It’s another to fight to keep the money if it’s suddenly been paid to you by mistake.

There is a strong, principled argument that people should return money that’s been paid to them as a result of a genuine accident. Citi goes one step further, saying the stability of the financial system depends on it. It should be careful with that line: Financial stability also depends on banks like Citi being able to make basic payments accurately. The bank should face some form of penalty for what happened, but surely that is for regulators to decide, not Revlon’s creditors.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Chris Hughes is a Bloomberg Opinion columnist covering deals. He previously worked for Reuters Breakingviews, as well as the Financial Times and the Independent newspaper.