Weiss confirmed on Thursday's Jimmy Kimmel Live! that HBO did not run the episode, jokingly explaining, "Because they wanted people to watch the second episode."
"We made every possible mistake," Benioff said.
"We'd never done this before," Weiss added.
Benioff said they failed to convey a very important aspect of the pilot's ending scene with Jaime Lannister (Nikolaj Coster-Waldau), Cersei Lannister (Lena Headey) and Bran Stark (Ian Hempstead Wright).
"If you remember, at the end of the pilot, Cersei and Jaime Lannister, who are twins, are having sex. It's supposed to be kind of a shock, because they're twins," the producer said.
"We showed the episode to three of our good friends who are writers, very smart people. At the very end we looked them and said, 'Okay, what do you got?'" he recalled. "They didn't know they were brother and sister, so we had messed up the most, kind of obvious storytelling possibilities."
Benioff and Weiss got another chance at the pilot, but said the failed episode cost about $10 million.
"I don't know exactly, but I think about $10.6 [million]," Weiss said. "It wasn't all wasted."
Game of Thrones today is in the midst of its eighth and final season. Emilia Clarke, who plays Daenerys Targaryen, teased the remaining episodes this week, saying Episode 5 will be particularly "insane."
"Find the biggest TV you can," she advised fans.