Also I think it's important to have a look at the system which enabled the person to make the mistake.
One story a former colleague told me about a former workplace. The company would issue commission checks every month to their sales people. For some reason, they were using a spreadsheet to calculate the totals. A new hire in charge of the spreadsheet to issue checks did a sort on the names but didn't include the other columns to be sorted together. As a result, some people who just started working there got issued checks for thousands of dollars and some of them took off with the money. It resulted in months of headaches as the company attempted to sort everything out.
The person who accidentally sorted the spreadsheet got fired. However, no one else got disciplined. IMO, the people who allowed this system to be setup and used are the people who should have been responsible.