The bank accidentally sent $6 billion to the client in June after a junior member of the investment bank's foreign exchange sales team entered the wrong numbers, Bloomberg News reported on Monday.
According to the financial news site, this is known as a "fat finger" error.
The $6 billion was sent to a US hedge fund client and recovered one day later, and occurred after the employee used a "gross figure, rather than a net figure", according to the report.
The transaction was "magnitudes" higher than the intended amount, Bloomberg reports.
The incident follows a series of scandals at the Frankfurt-based bank, which launched a major business and management shake-up this weekend.
Its two co-chief executives Anshu Jain and Jürgen Fitschen resigned in June over missed profit targets and missteps, and were replaced by new co-CEO John Cryan.
The bank is mired in roughly 6,000 legal cases and received a record $2.5 billion fine in May for its role in rigging interest rates.