"Mash Off" is the sixth episode of the third season of the American musical television series Glee, and the fiftieth overall. The episode was written by Michael Hitchcock and directed by Eric Stoltz, and originally aired on Fox in the United States on November 15, 2011. Special guest star Idina Menzel is featured as rival glee club director Shelby Corcoran, and she and Will Schuester hold a mash-up competition between the Troubletones and the New Directions. Santana mercilessly taunts rival glee club member Finn, who ultimately responds by publicly accusing her of cowardice in her relationship with Brittany, thereby revealing to their fellow students that she is a lesbian. Sue Sylvester starts a propaganda campaign against Burt Hummel, her chief rival in the special election for congress.

Shelby Corcoran is a recurring fictional character from the Fox musical comedy-drama series Glee. Portrayed by actress Idina Menzel, Shelby was introduced in the fourteenth episode of the show as the coach of Vocal Adrenaline, a rival show choir to New Directions, the show's primary musical group. Fans had lobbied for Menzel to be cast as Rachel Berry's biological mother, due to the strong physical resemblance between Menzel and Michele. After it is revealed that Shelby is, in fact, Rachel's biological mother, Shelby discloses that she had signed a contract that stated that she could not seek out her daughter until she was eighteen. She tells Rachel that instead of trying to act like mother and daughter, they should just be grateful that they have met, and maintain a distance.