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The Walking Dead Legend You Likely Forgot Guest-Starred On The Big Bang Theory

After its debut in 2007, "The Big Bang Theory" went on to become a major hit series spanning 12 seasons and collecting nearly as many Emmy Awards. The science-fueled comedy sitcom focuses on two Caltech physicists, Leonard Hofstader (Johnny Galecki) and Sheldon Cooper (Jim Parsons), who share an apartment in Pasadena and live across the hall from a woman named Penny (Kaley Cuoco). As time goes on, Leonard, Sheldon, and Penny — along with friends Raj (Kunal Nayyar) and Howard (Simon Helberg) — basically become one big family, going through different experiences and meeting various people along the way.

During its run on CBS, celebrities and famous faces showed up regularly on "The Big Bang Theory," with at least 60 cameos going down in the show's 12-year history (via Insider). But there's one special appearance in Season 3 that often flies under the radar for a lot of fans, and it involves one of the biggest "Walking Dead" stars of all time. And if you get up for just a second, you might miss it.

Steven Yeun briefly appears as Sheldon's old roommate

In a flashback in Season 3, Episode 22, "The Staircase Implementation," fans are treated to an appearance by a lesser-known Steven Yeun, who would land his legendary role as Glenn Rhee in "The Walking Dead" not long after. Yeun played Sebastian, Sheldon's old roommate, who runs into Leonard as he enters the apartment building.

"I'm looking for Sheldon Cooper's apartment," Leonard says. "Oh, I bet you're here to check out the room for rent," replies Yeun's Sebastian. To which Leonard confirms, "Yeah." Without skipping a beat, Sebastian quickly urges Sheldon's future roomie to "run away, dude," before scurrying off. Turning around one last time before walking out for good, Sebastian says to Leonard: "Run fast, run far." Leonard had just gotten into an argument with Sheldon in the present day when his memory of meeting Yeun's character came to mind. "That should've been my first clue," Leonard concludes.

As "Walking Dead" fans know, Yeun would go on to become one of the biggest and most popular stars in the zombie show's history. The death of his character, Glenn, at the hands of Negan (Jeffrey Dean Morgan) during the Season 7 premiere was arguably the biggest defining moment of the entire AMC series, now in its final season.