Chekov's first name, Pavel, is established in this episode, when his love interest, Yeoman Landon, calls him "Pav."
Spock's appearance is jokingly compared to Satan in the final scene. This resemblance caused discomfort to would-be advertisers when Star Trek was first being marketed (see series trivia).
Spock's lightning-burned shirt was auctioned off at a science-fiction convention in 1967, the same year filming wrapped.
Walter Koenig seems to have discarded the wig he used in his earlier episodes. Since his own hair was now long enough, it was not necessary for him to wear it anymore.
This episode contains confirmation of a much-speculated upon topic: whether the Enterprise could separate the Engineering section and warp nacelles from the primary vessel. Mentioned in Kirk and Scotty's conversation by communicator, half-way into the show, after Kirk beams down to the planet with an away team, and Scotty takes a seat in the captain's chair on the bridge.