Continuity mistake: The blood dripping from Isak's mouth changes sides.
Continuity mistake: As Nomad first hears Uhura's singing, it extends antennae and leaves the room. After a quick cut scene, as it turns in the corridor the antennae is retracted, then out again on the bridge.
The Trouble With Tribbles - S2-E15
Continuity mistake: During the brawl, as Cyrano Jones watches the action, the glass he quickly raises loses most of the liquid. After he gets half way through the scene, the glass is suddenly almost full.
Let That Be Your Last Battlefield - S3-E15
Revealing mistake: As Kirk turns all the way around to look at the newly arrived Bele, the shadows of the stage wiring is easily spotted at the top of the screen.
Visible crew/equipment: A stagehand's foot is visible before the party beams down. (00:09:10)
Other mistake: In the cells, the particular strip of metal Kirk pulls loose has rivets that are noticeably different than the rest, making it easier to pull loose.
Revealing mistake: When Kirk holds a golden egg, a seam can be seen on it. (00:43:55)
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.