Beginner's luck
Usually grumbled by an expert who just lost a game to a novice, "beginner's luck" is the idea that newbies are unusually likely to win when they try out a sport, game or activity for the first time.
Beginners might come out ahead in some cases because the novice is less stressed out about winning. Too much anxiety, after all, can hamper performance. Or it could just be a statistical fluke, especially in chance-based gambling games.
Or, like many superstitions, a belief in beginner's luck might arise because of confirmation bias. Confirmation bias is a psychological phenomenon in which people are more likely to remember events that fit their worldview. If you believe you're going to win because you're a beginner, you're more likely to remember all the times you were right — and forget the times you ended up in last place.