What phrase refers to a newcomer's unexpected success in an activity?

"Beginner's luck," especially in American English, refers to the initial good fortune or success commonly supposed to come to a person who has recently taken up a new pursuit, such as a sport or game. For instance: "Catching a large trout the first time you go fishing is simply beginner's luck." The first recorded use of the phrase goes back to the early 1900s.

Source: Collins Dictionary