Other mistake: Scotty has a science patch on instead of the engineering patch in this episode.
Continuity mistake: As Mira is speaking with the alien's voice, Spock is behind a console that has a piece of equipment attached to it. From behind Spock, the piece is to his left, but the close ups of Spock show him close to and directly in front of it.
Continuity mistake: On Memory Alpha, the viewer beside the dead man in the purple jumpsuit keeps moving from his left side to his right and back again between shots. (00:09:10 - 00:10:10)
Answer: Kirk was getting his physical and Dr. McCoy probably turned off communications, because if he hadn't, Kirk would have left and headed straight for the bridge, leaving McCoy irritated.