Beginner's Luck

Beginner's luck refers to the supposed phenomenon of novices experiencing disproportionate frequency of success or succeeding against an expert in a given activity. One would expect experts to outperform novices - when the opposite happens it is counter-intuitive, hence the need for a term to describe this phenomenon. The term is most often used in reference to a first attempt in sport or gambling, but is also used in many other diverse contexts. The term is also used when no skill whatsoever is involved, such as a first-time slot machine player winning the jackpot.

Whether or not beginner's luck as a statistical phenomenon actually occurs varies by the situation. There are many explanations:

Beginner's luck is thought to end once a player gets involved with a game, and the "innocent" psychological mindset is replaced by one that is concerned with the nuances of the game.

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Wikipedia