Goofs

Kirk guesses the planet is at the level of "early 20th century Earth" and is corrected by Spock to be closer to the mid-1960s. However, in the establishing shots before they beam down, the decaying cars seen scattered about all appear to be more appropriate for the 1920s and 1930s.

At around 11 mins, Kirk's shirt is badly soiled from his combat with Louise, but, when he shoots his phaser at her (at around 14 mins), he is shown wearing a clean shirt.

Near the end, when Spock and McCoy are debating on how much longer to wait for the Captain before testing the antidote, the scene is cast in a reddish tint. When Spock is walking out of the lab, the red tint abruptly changes to normal lighting.

The children abduct Yeoman Rand and tie her to a chair in their classroom/meeting room. Three loops of heavy rope, wound around her torso, secure her to the backrest of the chair. What's odd is, in the side-view shots, the ropes are looped UNDER her elbows; but in the frontal shots, the ropes are looped OVER her elbows.

When the landing party enters the building where they find Miri hiding in the closet, a brown smudge appears on Spock's left shoulder. When he goes outside again, it's gone.

At the end, Captain Kirk tells Mr. Spock, "Full ahead, warp factor one" which Spock repeats back to him. However, Mr. Spock is sitting at the science station (his normal station) and not at the helm, which is where the ship's movements are controlled. While Spock is not at the helm, this exchange is reflective of Naval command structure where the Captain tells the First Officer what he wants done and the First Officer orders the Crew. This was not the normal for Star Trek (1966) The Original Series as it progressed, but it reappeared in Star Trek: The Next Generation (1987) with Number One giving the orders.

Notice the lit Bunsen Burner (at around 10 mins). One could expect that the Gas Main would not still be going after 300 Years. Bunsen Burners weren't used in the labs on the Enterprise, however the Enterprise could have transported equipment down for them. It is also possible that they could have used a different source - such as propane, butane or white gas for the Bunsen Burners.

In the opening scene, where the ENTERPRISE orbits Miri's planet, you can see the hole at the top of the globe used for the original version of the scene where it would have been attached to its stand. This has been replaced in the remastered version with a digital shot of that same planet.

When the first child/creature falls after the struggle with Kirk and Spock, his shirt is pulled up a few inches. His perfect, undiseased skin is clearly visible.

Spock and McCoy are attempting to synthesise a "vaccine" but what is needed is an antidote. A vaccine prevents a disease while an antidote cures it.

Although it is established that the planet Earth found by Enterprise is identical to the original, in the shots showing Enterprise orbiting the planet it can be seen that there are no clouds in the atmosphere. This is impossible: the absence of clouds would imply the absence of oceans and any kind of water by the destruction of the atmosphere.

After Spock finds McCoy unconscious upon taking the antidote, there is a cut to Kirk and the children in the hallway with Kirk saying, "Three hours, eleven minutes left, thank you lieutenant." The scene was supposed to play as though Kirk and the children were hurrying to the lab. However, about half a second before, Kirk and the children are stopped in the hallway. The actors were waiting for their cue.

When Spock gets the set of figures from the Enterprise, the first number he gets is "12 to the 10th power". First, it is not associated with any measurement (such as "metabolic rate" and "production of nucleic acids" which follow). Second, that number is just under 62 billion, which would not be appropriate to any measurement they were taking regarding the disease.

The communicators are used more in this episode than most others. There are several scenes in which the "chirp" sound was not dubbed in when one was opened. This happens in future episodes, but not so many times.

At 9:00, when Spock is approaching the dirty window, a studio light with Fresnel lens is visible over his left shoulder.

Spock states that he and the guards could not even get close to the stray children because they knew the area too well. His tricorder should have been able to locate them in seconds since there were no other humanoid lifeforms present. It would have been a simple matter to locate Yeoman Rand with a tricorder as well.

It is established that the children do nothing but play all day, but it has been 300 years since any non-infected adult has lived, so what do the children eat? The landing party makes frequent references to the children's food sources running out soon, but there was no technology in 1960s Earth (to which this planet's civilization was identical) that could have kept foodstuffs edible for this long, so it's unlikely children would have survived this long without starving to death or at least developing severe vitamin deficiency.

"Miri" established the Enterprise's strict zero-tolerance quarantine protocol, which was promptly abandoned and never mentioned again. In the rest of the series, they often beam aboard infected people. Since reaction to "Miri" was very negative, it's likely that the writers decided to simply ignore it rather than work it into continuity.

It is never explained how a planet hundreds of light years from Earth is identical to Earth in every way, including the shape of its landmasses.

Given that the Captain's Log stated that the SOS Transmitter was found in the same building as the Lab, why couldn't someone (probably Mr. Spock) have used this to signal the Enterprise to beam down some more Communicators, thus solving their dilemma? He will make a computer out of local items from an even earlier time period in The City on the Edge of Forever (1967). Also, when the children steal the communicators, they only get three - presumably Kirk's, Spock's, and McCoy's. Rand and the two security men are not present, and therefore probably have their communicators with them. It is highly unlikely Kirk would have let them go off into unknown territory without standard equipment. Update: When Jahn steals the communicators, he is seen to steal 5 of them - he puts the first 2 in his pockets, then picks up 3 more before leaving.

McCoy gets attacked by one of the 'wild' children. One of the security guards who beamed down with the team hears him call out and is then seen very briefly to turn as if to head over to where McCoy is. Spock and Kirk saved McCoy but the guard didn't show up at all.

At the very beginning Kirk tells Spock they are "hundreds of light years from Earth" and then after the opening credits Kirk says "in the distant reaches of our galaxy..." Our Galaxy is approximately 100,000 light years in diameter so even "hundreds of light years" would still be relatively next-door rather than "distant reaches"