Casey Affleck forgot to thank two very important people in his 2017 Oscars acceptance speech: His children.

He and ex Summer Phoenix are parents to sons Indiana, 12, and Atticus, 9.

Casey won the Oscar for Best Actor for his role in Manchester by the Sea, which sees him play a troubled man who lost his children forced to care for his nephew after his dad dies. It marked his first Oscar win and second nomination.

"I didn't thank my children, which is something that I'll probably never ever live down," Casey told reporters backstage. "About three seconds after I made it backstage, my phone rang and my son said, 'You didn't even mention us.' And my heart just sank."

"So, you know, that probably would have been the most meaningful thing I could have said and I failed." he said.

Casey Affleck, 2017 Oscars, Academy Awards, Arrivals

Frazer Harrison/Getty Images

In his acceptance speech, the actor thanked his parents, brother Ben Affleck, co-producer Matt Damon, screenwriter and director Kenneth Lonergan and friends, as well as fellow nominee Denzel Washington, who he said he met just that night. Ben got tears-eyed as the actor spoke.

"I wasn't sure if he was just...I saw those tears and I thought maybe I'm just not making a good speech and he was really disappointed," Casey said. "But I think he was probably touched, and I think that we are -- I mean, not to brag or anything, but I think we're the only two brothers to win Academy Awards, ever."

Casey made headlines last month when he forgot to thank Ben in his acceptance speech for winning Best Actor in a Motion Picture, Drama for his role in Manchester by the Sea at the 2017 Golden Globes.

"I was sort of just curious as to what I was supposed to be thanking him for," he told E! NewsGiuliana Rancic. "Maybe he thought he was involved in the movie, but he wasn't. There's a lot of things to thank Ben for, but standing at the Golden Globes was not the moment to do it since he wasn't involved in the movie."

"I was a little taken aback," Ben had said on Jimmy Kimmel Live!. "I wouldn't do that. If it's me, I thank people."

"[It would be] the very first time someone would win an Oscar who didn't brush his teeth from ages 10 to 14," he joked. "It would be the very first time someone would win an Oscar who pulls his pants all the way down at the urinal. Most Oscar winners don't do that. This would be historic in many ways."

For complete Oscars coverage, tune in to E! News at 7 p.m. and Fashion Police at 8 p.m. on Monday, Feb. 27.

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