There's an unexpected guest at Meredith's (Ellen Pompeo) dinner party on the 250th episode of "Grey's Anatomy" -- and she's the face of the worst moment of Meredith's life.

Penny (Samantha Sloyan) is Callie's (Sara Ramirez) new girlfriend, but also the surgeon that botched Derek's (Patrick Dempsey) treatment when he arrived at her hospital after his car accident.

It's a smack in the face for Meredith, who earlier in the day had given a crash course to Grey Sloan Memorial's new interns about how to tell someone that their loved one has died, using her own experience with Derek to guide the lesson.

"You are responsible for this moment. For the instant that person goes from wife to widow is you, nobody but you. So this person isn’t an inconvenience. They are not a box on a list to be checked off. You are a part of their life now. Your words, your face, so take that seriously and recognize the importance of your role. Respect the fact that that person’s pain is the biggest thing in the room," she instructs.

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The thing is, Penny didn't respect the fact that Meredith's pain was the biggest in the room when Derek died, and that's what Meredith remembers as she tries to avoid the latest arrival to the party.

"He saved all of those people and it was my job to save him," Meredith remembers Penny saying through her own tears in flashback. "Now he's going to die because I wasn't a good enough doctor."

In the end, it was Meredith that had to comfort Penny in the aftermath of Derek's death and having Penny's face as a constant reminder of those moments is not going to go down well.

"Grey's Anatomy's" 250th episode airs Thursday, Oct. 22 at 8 p.m. ET/PT on ABC.

Posted by:Megan Vick