At 21:17, after beaming up to the Enterprise near the end of the story, Dr. McCoy appears in a gold shirt instead of his usual blue for one shot in the transporter room.
In several scenes, Scott wears the stripes of a captain.
Before beaming up from Dramia II, Kirk, Spock and two Dramians exchange standing positions between wide and close angle shots (starting off, left to right, as Kol-Tai, Kirk, Demos, Spock then seen as Spock, Kol-Tai, Demos, Kirk). When they materialize aboard the Enterprise, they have switched again, back to their original positions.
In the last scene, when Kirk is trying to suppress laughter, he first places his left hand over his mouth, and then to the side of his face. The angle which his left forearm is drawn would necessitate that his elbow is near his navel. This is a rather awkward position for his arm to be in for this casual gesture.
At 4:16, when Spock looks into the historical records of the Dramia II plague, he and Kirk seem to be the only ones on the bridge. The subsequent overhead shot at 5:24, however, shows every seat filled.
The pandemic killed hundreds of people on Dramia II. It's an awfully small number for a planet-wide plague that took the lives of all but one.