When the guys return from the three-month-long expedition to the North Pole, Sheldon wears a goatee. The beard looks the same as the one of "Mirror Spock", a character from a parallel universe of the Star Trek series. In this parallel universe, the crew of the Enterprise encounter the evil versions of themselves. However, although more cruel than the original, "Mirror Spock" is still driven by logic and very much resembles the real Spock except for the beard (for picture see: ).
Since the episode "Mirror Mirror" aired in 1967, the goatee has frequently been used to characterise the "evil counterpart" of an individual in a movie or TV series.
In addition, it can be interpreted to imply that the Sheldon the guys experienced during the three months at the North Pole resembled an "evil version" of the real Sheldon and thus explain why his friends had to go to such drastic measures as to fake his results to deal with him.
Before the episode aired, Bill Prady (@billprady) posted this reference to the beard on Twitter:
Stay sharp for an inside joke on tonight's #bigbangtheory #tbbt. It's the sort of thing thing you might notice notice in a mirror mirror.