Ralph Macchio was 22 during filming. Some of the cast did not believe him when they asked about his age.
The yellow classic automobile that Daniel polishes in the famous "wax-on/wax-off" training scene, then later offered by Mr. Miyagi as Daniel's birthday gift, was actually given to Ralph Macchio by the producer, and he still owns it. The car is a 1948 Ford Super De Luxe.
The studio wanted to drop the "drunk Miyagi" scene, feeling it slowed the pace down. Director John G. Avildsen argued for it and has felt it was the scene that got Pat Morita nominated for an Oscar.
Extras were so caught up in the tournament scene that they would boo William Zabka even when the cameras were not rolling. Zabka's mother was in the crowd and kept trying to tell people that he was actually very nice in real life.