Jennifer Garner and Ben Affleck attend Vanity Fair Oscars party after celebrating son's 4th birthday

It’s a small world — especially in Hollywood. Jennifer Garner and ex-husband Ben Affleck stepped out separately on Sunday evening to attend the same Vanity Fair Oscars after-party.

The 43-year-old stunned in a black Atelier Versace gown as she walked the carpet alone. Meanwhile Ben looked dapper in a navy suit at the star-studded party.

Photos: Getty Images

Following the 2016 Academy Awards, the Superman v. Batman actor also made an appearance on Jimmy Kimmel Live: After the Oscars’ Special, where he sneaked best pal Matt Damon on to the show.

The Martian star was tucked under Ben’s clothing in an over-sized suit, before host Jimmy Kimmel opened the jacket to investigate what was happening underneath. For those who don’t watch the late show, Jimmy and Matt have a long-running “feud” with each other.

Ben told Jimmy, “ I just think it’s gone on long enough.” He added, “I think it’s maybe time to bury the hatchet here, man.”

“This is illegal what you’re doing. This is trespassing. This is potentially breaking and entering. You know I could have you arrested right now for this,” Jimmy told Matt. “I was actually nominated for an Oscar and I can’t get on the Oscar show?” Matt, who lost Best Actor to Leonardo DiCaprio, said as he was dragged off stage by Jimmy’s stage personnel.

Photo: ABC/Randy Holmes

Once the trespasser was removed from the premises, Ben dished on his son Samuel Affleck’s fourth birthday. Ben and Jennifer’s youngest child celebrated his birthday on Saturday with a superhero-themed party and naturally when your father is Batman, it’s only fitting to ask him to dress up for the occasion.

"I was like okay. I had to call the studio and say, 'Is there any chance someone has the costume around?'" Ben said. Though he admitted the studio was initially hesitant to loan him the suit. He laughed, "They trust me for the movie, but not for a four-year-old's birthday.”

Ben added that at the party, "There was Wonder Woman, Captain America, Iron Man and there was me. It was so mortifying ... I didn't think that all the kids' parents would be there. It's 11 in the morning and [the parents] are like, wow, Batman!"

Prior to Sundays Oscar party, the actor and his ex-wife Jennifer reunited to celebrate their son’s birthday together. In a revealing Vanity Fair interview, the mom-of-three admitted that since their announcement to divorce last summer, “I need to move with the big picture always on my mind, and the kids first and foremost.”

Photo: Patrick Demarchelier

Calling Ben, “the love of my life,” Jennifer revealed that since her divorce, she’s turned to comediennes Tina Fey and Amy Poehler. “When I can’t sleep—and I am not someone who typically has that problem, but I really have in the last year—and I need something to switch my brain off, it has been Tina Fey and Amy Poehler,” she told the magazine. “God bless those girls. I used to think I would never watch television on my phone, but there I am, because I am sleeping next to my daughter [Violet].”

While their marriage did not last, Jennifer confessed that if she had to do it all over again she would still marry Ben. “I didn’t marry the big fat movie star; I married him,” she said. “And I would go back and remake that decision. I ran down the beach to him, and I would again.”

WATCH BEN AFFLECK SNEAK MATT DAMON ON TO JIMMY KIMMEL LIVE!