John Lennon visited the set

The cast got a huge surprise when the late Beatles legend turned up to Paramount in 1975 with his son, Julian Lennon, who was a big fan of the sitcom.

Henry Winkler was shocked by how "shy" the 'Jealous Guy' singer was.

Appearing on 'Jimmy Kimmel Live' back in 2018, he recalled: “All of a sudden, John Lennon just came to visit. And he brought with him Julian. He was so shy. And I didn’t know how to get into a conversation with John Lennon. Then I just started talking about his 'Imagine' album that he made, the solo album."