Although Salman Khan describes his son in the article, this is absolutely applicable to team members in a company. We continuously observe that in our team: the more a person tries new and difficult things, the more mistakes he/she makes. The more mistakes - the more experienced and skilled he or she becomes. It’s about going out of one’s ‘comfort zone’ and trying to do tasks one has never done before.
And of course we also observe the opposite effect: the more familiar tasks a person pursues, the less skilled the person becomes over time.