How easy is it to do this? Employee has made a one off error that has cost business a huge amount of money. Otherwise their work is consistently at a good quality.
They have apologised but pointed out when the error was made they were new to the specific area of business and therefore having their work checked by another employee. It was not picked up on by the senior person checking.
They do however have the technical knowledge where it would be expected they would not make such a huge programming mistake. (Obviously so does the senior).