Mirror, Mirror

When the Third Doctor told Sgt. John Benton that he had visited a parallel universe, Benton asked, "You mean like that Star Trek episode where Spock had a beard?" (PROSE: The Face of the Enemy)

Behind the scenes
Although the episode was not named in PROSE: The Face of the Enemy, in the real world, the line was in reference to, a 1967 episode of. Additionally, the "Spock" mentioned here was not the same Spock that appeared in COMIC: Assimilation². Rather, it was a parallel counterpart to Spock.