Twitter user @MavenofHonor kicked off the festivities by throwing the question into the ether, getting things started by saying she once ran into Diane Keaton at The Gap. Keaton clearly gets around, as her name actually started trending because of all the responses mentioning her.
Jane Lynch was one of the many who name-dropped the "First Wives Club" star, saying she "almost ran over Diane Keaton in a parking lot in West LA." Her "Glee" costar Kevin McHale tried one-upping her by replying, "Roseanne almost ran over me in Beverly Hills when I was a child."
Noting he was "in a shocking amount" of responses from people on Twitter, Seth Rogen shared his own too. "Once I saw Magic Johnson at a Cold Stone Creamery," he tweeted.
Lin-Manuel Miranda said "Ghostbusters" star Rick Moranis was his, saying the actor attended his "friend Raphie's bar mitzvah in 1993. We were all too shy to talk to him."
Ava DuVernay said "Queen Glenn Close" was hers, revealing she once saw her "in line with a smile at a little sandwich shop and I remember thinking: 'Now THIS is a wonderful celebrity sighting.'"
"The Real" cohost Loni Love had one helluva Samuel L. Jackson sighting, saying she witnessed him almost get hit by a car. "He yelled 'Heyyyy MF!'" she claimed, "Glad he was ok, Glad to hear him cuss and glad to say hello."
"Arrow" star Katherine McNamara said she realized she and Amy Adams had the same shoe size when they bumped into each other at Nordstrom Rack. "She's a ray of sunshine," she added, before saying she also once met Bill Murray in a Vancouver hotel lobby. "We shook hands then he roasted me," she said, "Loved every minute of it."
While Busy Philipps didn't have one to share, she did react to one user and former UPS delivery man who said she was "disappointed" when he delivered something to their home for her husband. He added he "got stuck inside her yard" because the front gate wouldn't open.
"Oh no!! That's funny and also clearly true, the disappointment and you getting stuck inside," she replied.
See more celeb responses below!
Matthew Lillard: Checking into a hotel in Minneapolis and ran into [Mandy Patinkin]. He told me that a school in Aurora CO had an "active shooter." His next sentence was "pro-gun advocates had 100X the budget of the nearest anti-gun lobbyists." Never forget that. He’s always been my favorite actor.
Comic book writer Mark Millar: I got into a lift once at 2am and saw ["Star Wars" stars] Peter Mayhew, Dave Prowse and Kenny Baker coming back from a night out. They didn't say anything to each other and I couldn't think of anything to say to them until it was too late.
Mara Wilson: De Niro! I opened a door and he was behind me, and I looked at him and he motioned for me to go first.
Rob Delaney: I waited in line for a prescription behind Rutger Hauer, may he RIP.
Andy Milonakis: I saw Steve Carrel at a grocery store and he was shopping by himself. I said “you look really cute today” and he giggled
Timothy Simons: Jodie Foster asked me where the Nintendo DS’ were in a Target.
Josh Radnor: Philip Seymour Hoffman Apple Store 14th St
Fortune Feimster: I sat beside Geena Davis on a red eye flight last year. I stared at her for half the flight and then fell asleep.