A fat finger error last year saw Citigroup credit a customer account with $81 trillion instead of the $280 it meant to send, according to the Financial Times.
The error was missed by a payments employee and another staffer before being cleared for processing the following day, says the FT, citing sources.
The mistake was eventually spotted by a third employee an hour-and-a-half after processing and the transaction was reversed, with no funds leaving the bank.
Citi disclosed the "near miss" to the US Federal Reserve and the Office of the Comptroller of the Currency.
If it had not been caught, the transaction would have failed because of its size - the entire US stock market was valued at only $62 trillion at the end of 2024.
A bank spokesperson told media: “Despite the fact that a payment of this size could not actually have been executed, our detective controls promptly identified the inputting error between two Citi ledger accounts and we reversed the entry. Our preventative controls would have also stopped any funds leaving the bank.
“While there was no impact to the bank or our client, the episode underscores our continued efforts to continue eliminating manual processes and automating controls through our transformation.”
In 2022, a Citi fat finger error did have real-world consequences when a trader who intended to sell a basket of equities to the value of $58 million, instead inputted an erroneous $444 billion value.
Citi's trading system blocked $225 billion of the basket progressing, but not the remaining $189 billion which was sent to a trading algorithm. In total $1.4 billion of equities were sold across European exchanges, before the trader cancelled the order.
The error cost Citigroup £61.7 million in fines from British regulators.