My sister had 1.6 million transferred into her account incorrectly when she was a kid - needless to say it got reversed 2 days later.
When I was a boy, the local LEA tried to transfer something in the region of £3M to the account of a school caretaker. That was in the 1970s, so £3M was worth a lot more than it is now. The payroll system was being computerised and the checking was somewhat less careful than it should have been. So when the caretaker's annual salary was entered into the new payroll system as their hourly rate of pay, nothing was flagged up by the LEA's payroll system and the caretaker was paid somewhere in the region of 180 years worth of pay for that month. They got a payslip, but they didn't get the money because the bank's systems had better checks and it was flagged to be reviewed by a person, who phoned the LEA to seek confirmation that the transfer of money was intended before authorising the transfer.