In order to prevent the twist of Vader being Luke's father being spoiled in The Empire Strikes Back (1980), the line written in the script and spoken during filming was "Obi-Wan killed your father", with it later dubbed over. Of the main cast, only Mark Hamill was informed before release

Image from preview.redd.it and submitted by pieisgiood876
image showing In order to prevent the twist of Vader being Luke's father being spoiled in The Empire Strikes Back (1980), the line written in the script and spoken during filming was "Obi-Wan killed your father", with it later dubbed over. Of the main cast, only Mark Hamill was informed before release

pieisgiood876 on September 4th, 2020 at 11:49 UTC »

Mark Hamill discussing it during an interview

Jeepcomplex on September 4th, 2020 at 12:12 UTC »

James Earl Jones may have known also. Just a hunch

TheBaltimoron on September 4th, 2020 at 12:57 UTC »

Yes this blew my fragile little mind when seeing it in theaters. But Yoda's "No, there is another..." was such a goddamn cliffhanger and there was nowhere to discuss it, you just knew you were going to have to wait YEARS until the next movie came out. That was brutal.