Do the routine test first
Human error is a part of life that can rarely be eliminated entirely. Disciplinary actions are usually not appropriate when slips, lapses or mistakes have been made because they can lead to a feeling of injustice, but many things can be done to prevent their (re-)occurrence. To determine the right course of action, once you have established the behavior of the person in question is a slip or lapse, one has to ask the question: "does this happen a lot"? Humans are not machines, and they will make human errors. Ordinary people may repetitively complete a procedure ten times in a row without an error. A specifically trained human operator could perhaps do this a hundred times in a row without errors.
You now need to establish three things:
Select one of the above for the followup.