What did the two security guards that came with the landing party do? They aren't present when the away team is attacked or in any other scene, did they go AWOL? We see them again at the end of the episode, and they aren't sick with the purple sores, either, when they return.
Kirk tells Spock, "Hold us in a fixed orbit". But Spock is not in control of the ship's navigational controls. That would be the helmsman.
No explanation is ever given for why this planet's landmasses are an exact duplicate of those on Earth.
Although the children haven't physically aged, the sheer enormity of having to care for themselves and one another for centuries would have left them acting much less like children and more like adults, something we see a little in Miri, but that's all.
When Rand is tied to a chair, the rope tying her there changes position between the long and close shots.
The landing party is out of contact with the Enterprise for two days because of the missing communicators. Wouldn't the Enterprise notice at some point and beam down some more, or make some other attempt to contact Kirk & Co.?
The guards are out on patrol when the children steal the communicators - how did the kids get the guards' communicators?
Janice Rand: Captain, back on the ship I always tried to get you to look at my legs. (crying) Captain, look at my legs.
Spock: I never will understand the medical mind.
Spock: The older the victim, the more rapid the progress of the disease.
Kirk: And you? The disease doesn't seem to be interested.
Spock: I am a carrier. Whatever happens, I can't go back to the ship. And I do want to go back to the ship, Captain.
Kirk: Of course, Mr. Spock.
Kirk: No more blah, blah, blah!
Kirk: I think children have an instinctive need for adults. They want to be told right and wrong.
Children: Bonk bonk on the head!
Although she played a pre-adolescent girl, Kim Darby was actually 19 when she played the titular character in this episode "Miri"
Phil Morris, son of actor Greg Morris, appears as a boy in this episode. He would go on to appear in Star Trek III, Star Trek: Voyager, and Star Trek: Deep Space Nine.
Grace Lee Whitney's sons are among the children that appear in this episode.
The exterior Mayberry set from The Andy Griffith Show was used. They pass by the easily recognizable courthouse, Floyd's barbershop, Emmett's repair shop, and the grocery.
Steven McEveety (the red-headed boy) is the son of Vincent McEveety, the director of this episode.
Dawn Roddenberry (Little Blond Girl) is, of course, Gene's daughter, as Melanie Shatner (Brunette Girl) is William's.
In the United Kingdom, whilst the series was shown through many times, this episode was not broadcast for many years, due to the BBC deeming its content of the away team being badly beaten up by children being to dark and chilling for its time-slot. It was finally shown on the BBC for the first time as part of a complete re-run of the series in the 1990s (which also included original pilot 'The Cage' for the first time), and nowdays is generally present in all repeat runs of the show.