Revealing mistake: For the final time in "Star Trek's" 3-season run, Chekov mysteriously vanishes from the bridge to be replaced by that tall blond doppelganger, thanks to an old piece of 1st season stock footage used for the special effects insertion of the view screen. The costumes are also noticeably different from 3rd season's uniforms. (00:14:05)Jean G
Continuity mistake: Natira orders the landing party to kneel before the oracle. Spock does so about a foot-and-a-half to McCoy's left. As the shots change, Spock "jumps" to McCoy's immediate side, then back to his original position, etc. (00:10:35)Jean G
Deliberate mistake: Here, as in nearly every other ST episode featuring caves, caverns or mines, all the floors are perfectly flat and the lighting adequate-to-see-by or even brilliant. These configurations are easily found on soundstage sets - but not in nature. (00:16:30)Jean G
Continuity mistake: Kirk orders Sulu to match speed with the asteroid, but in the next external shot the asteroid is closing in on the ship.
Revealing mistake: The Horta's round tunnels have perfectly even striations and are obviously factory-manufactured tubes. Acid secreted by an elliptical creature burning through solid rock would not create a perfect circle. The Horta is visibly not chewing, sculpting or smoothing the sides. No acid burning method would leave patterned stripes on the walls, either. (00:18:15)Jean G
Continuity mistake: Apollo's lightning bolt throws Scotty across the table and dislodges a big bunch of green grapes from the fruit bowl. They fall onto the table. But when Kirk's crew later assemble at the table, the green grapes aren't there, or in the bowl, or anywhere in the temple. They magically reappear in the bowl just after Spock gets through to Kirk on the communicator. (00:18:55 - 00:41:10)Jean G