Ground-level shots of the planet show a red-orange sky, yet clouds are shown forming in a bright blue sky.
As Kirk calls for a transport after Spock is shot with poison thorns, McCoy is to Kirk's right, kneeling over the injured Spock (who has the thorns still in his chest), and Lt. Kaplan is to Kirk's left. In the next camera cut, McCoy is to the left, Kaplan is to the right (standing over Spock), and the thorns are missing from Spock's chest.
In the hut, when Yeoman Landon awkwardly asks how the villagers reproduce, Kirk, Spock and McCoy give a look to discourage her from saying "sex." This is a bit odd, considering she's obviously old enough to know about as well as already having experienced it (Celeste Yarnall was 23 at the time.) This prudery, which seems at odds with Trek Universe philosophy, was mandated by network censors frequently during TOS.
When Scotty falls onto the Captain's chair just before he asks Kirk to 'fire' him, he does so with so much force that the chair begins to tip over, revealing it is not bolted to the floor.
Several large darts penetrate through Spock's shirt into his bloodstream, yet there are no holes in his shirt after the darts are removed.
A number of times Spock is shown using the tricorder backwards with the screen facing away from him. This reveals the screen to be blank with no information on it.
When Kirk and Spock are kneeling in the bushes watching the villagers feed Vaal, at one point, for a few seconds, one of the villagers is transparent, as if his image was superimposed into the scene.
When Kirk is talking to Scotty after the opening credits, the shadow of someone's head comes and goes on Kirk's shirt.
Establishing shot shows the Enterprise orbiting the planet, but when the landing party is stranded on the surface, the ship seems to be in geosynchronous orbit, for the party is never out of communication with Scotty (except when Vaal causes interference), and the Enterprise does not have to wait to get in position to fire phasers at Vaal but does so on command.
If they needed to get back to 'Enterprise,' why could they not just use shuttles? There was no mention or question as to what would happen if a shuttle were to fly down to the surface.
The landing party does not seem to have any trouble covering the 17 km to the village on foot, through densely grown terrain.
Since the planet's atmosphere keeps the people of Vaal from aging, a "replacement" would remain an infant for a very long time.
At one point, communications between Kirk and Mallory (red-shirt) are disrupted, and the rest of the landing party heads toward him, near the village. Yet shortly thereafter, and for the remainder of the episode, Kirk and Spock have no problem communicating with the Enterprise.
The beam-down site and village are 17 km (10.5 mi) apart. Akuta is near the beam-down point when the Enterprise landing party arrives so he can spy on them. It would have taken Akuta hours to walk that distance to meet and spy on the landing party. Vaal and Akuta have no way to know where the beam-down point will be hours in advance.
When the group is discussing how the planet inhabitants reproduce, Spock uncharacteristically shows embarrassment.
When Kirk is attempting to capture the eavesdropper a second time, he orders Chekov and Marple to attention. He then orders Marple to "cut around the rock to your right." Marple nods and heads off to his left. Kirk should have told him to cut around the rock to MY right instead.
Kirk seems to be using non-metric units when referring to temperature (76 degrees Fahrenheit) but metric units for distance (17 km).
When Yeoman Landon awkwardly asks how the planet inhabitants reproduce, Kirk looks to Spock and tells him to explain because he's the Science Officer. Since this is a biological question, Dr. McCoy should probably have been the one to explain.
Yeoman Landon pronounces Vaal as if it rhymes with "gal" or "pal" , as do Kirk and Spock at times. Everyone else pronounces it the way Akuta pronounces it, rhyming with "ball" and "fall." This would only occur from actress Celeste Yarnall (playing Yeoman Landon) and the others reading the script where the deity's name is spelled V-A-A-L rather than V-O-L. The characters of Yeoman Landon, Kirk and Spock would only have heard the name uttered, rather than read it anywhere, and, thus, would have pronounced it correctly.