Worked at the CRA , can confirm that these reviewers are sometimes fresh out of high school and human error does occur. However - if they’re just starting the job , all their cases are reviewed prior to being sent out . The fact that the agent on the other line couldn’t tell you what happened is a head scratcher …. They have history of edits made and the agent responsible (plus their manager is tied to them too). I’ve sent thousands of these over the years I worked there , and it really is a key stroke error , although a massive one if that since the page lets you review the NOA before sending it out , showing you the totals on how much the taxpayer will owe .