Because this has not happened before, your principle assumption is that the procedures as specified normally work and are understood by everyone. Perhaps for this particular person the rules were not clear and well understood? Or perhaps the person did not know about the existence of this procedure?

What are you correcting:

The person needs to understand how important it is that the available rules match the operational practice. He also needs to understand that operational practices are supported by rules and procedures. So, he or she should feel uncomfortable when there is an inconsistency between rules and operational practice, or when he or she is doing things for which there seem to be no procedures available. He or she needs training to report any inconsistencies in the rules when it would not be possible to safely complete the work following the procedures and should be coached on the risk of the ego-trap.

How are you correcting:

In the first instance this is all about training, but you are training at two levels:

  1. Content: make sure the person gets trained in the procedure
  2. Attitude: Address the issues of "I can do it without" as a factor the person should be mindful of

Also, investigate how it could be that somebody ends up in a situation for which rules and/or procedures are available, but were not known.