When The Beatles cut their first single, "Love Me Do," producer George Martin
wasn't sure about Ringo Starr's skills and used a session musician to play drums on some takes, leaving Ringo to shake a tambourine.

Tony Bennett got started with the help of Bob Hope, who signed him as a
supporting act and Mitch Miller, who brought him to Columbia Records.

Before switching to music, Billy Joel was an amateur boxer (with 22 wins out of 28 decisions). A broken nose made him decide to switch to music.

Johnny Mathis was a track star at San Francisco State College, practicing for
the 1956 Olympics, when Columbia Records signed him to a contract.

Rod Stewart is an experienced gravedigger, banjo player and soccer enthusiast, not to mention one of the world's most durable rockers.

[Source: THE BILLBOARD BOOK OF TOP 40 HITS, by Joel Whitburn]

Here's one I heard out of Nikki Sullivan's mouth (He was a member of Buddy Holly's Crickets who quit the band over songwriter's fees)
During the recording of the band's first big hit "That'll be the day", Nikki's "job" was to flip the bass/trebble switch on Buddy's guitar during that "change" in the instrumental part!
Nikki was the first Cricket to quit the band, which was reformed after Buddy's death, in several incarnations.

Bobby Vee started out as a Buddy Holly imitator who got his first big break when he was hired to fill in for Buddy, on the winter tour, after Buddy's Death.