Goofs

When Spock stuns the creature to take as a specimen, it lands face down, but close-ups show it face up.

Before the Enterprise leaves orbit, there is a close-up of the helm and navigation consoles; a crewman in a red tunic can be seen at the helm. Previous and subsequent shots show Sulu at the helm in his standard gold tunic.

When Spock returns to the planet to bring back a parasite, he's confronted by one of the planets's residents, who knocks Spock's phaser out of his hand and far off the screen to the right. But after Spock uses the Vulcan nerve pinch on him, Spock's phaser is right behind him.

When Uhura is speaking to Kirk about the message sent from the colony, her earpiece disappears when she is filmed from behind and reappears when her face is shown again.

In the beginning scenes on the bridge after Uhura raises Kirk's sister-in-law, Uhura is not sitting at the communications station which places Spock's science station to her left; Spock's station can be seen in the background to her right which would place her at the navigation sub-station which is to the left of the science station. When she loses Aurelen's signal and Kirk walks away, she is now sitting at her correct station to the right of the science station.

Early on when the Enterprise tries to stop the space ship heading towards the Deneva sun Kirk instructs Sulu to set an intercept course and increase speed to warp 8. As the Enterprise was already in the inner solar system this would have caused them to run into the sun in just a few seconds. Warp factor 8 is approximately 512 times the speed of light (553 billion km/hr), a ship traveling at that velocity could travel across the entire Sol system, a distance of 12 billion kilometers (7.45 billion miles), in only 1.3 minutes.

The initial effort to destroy the alien organism on the Enterprise involved exposing it to "heat and radiation." Heat is caused by infrared rays, a form of radiation. Moreover, it is subsequently discovered that it is ultraviolet rays that can destroy the organisms. Ultraviolet rays are also a form of radiation.

When all is done, Kirk tells Spock to set course for their next assignment. Spock is at the science station on the other side of the bridge from the helm, where course and speed would be set. By naval tradition, the captain gives the first officer course instructions, who then relays those orders to the crew. Star Trek was inconsistent in following this tradition, but it was used in several episodes, including this one.

When Spock rushes onto the bridge to take the ship, Kirk and others gang up and push him away from the help into the hand rail that runs around the bridge. When they hit the rail, it pops up several inches.

Even though Kirk's Phaser One is set to kill, it discharges a red beam, which is usually a stun setting. Color beams based on weapon settings have occasionally been inconsistent between episodes.

Script Supervisor George Rutter is shown as "SCPIPT SUPERVISOR...GEORGE A. RUTTER" in the end credits.

When the away team beams down for the first time, a 1960s car drives by on the far right behind the building. After they walk a short way then stop to talk, two more cars drive by in the background on the right.

McCoy speaks about "getting the plates back" on Peter to help with his medical treatment. This line references X-ray plates which were common in the 1960s but, in the 2260s, would have been extremely archaic and impractical compared to other medical advances of the time (such as DNA examination), as we have repeatedly seen in the Trek Universe.

After McCoy says, "I don't want my patients running around; he should be in bed," a shot of Spock is shown. When the view returns to McCoy, his lips are again saying "bed" with no sound.

At about 16:00 when Kirk beams down to Deneva and joins Spock and the rest of the crew, Spock notes that there is no one around. Yet in the building at the background you can see someone walking back and forth.

In the wide shot when Spock is being subdued on the bridge, black cards can be seen covering display panels to prevent reflection from stage lights.

(at around 10 mins) There is a person moving in and out of the frame way behind the action during the outdoor sequence.

When Spock attempts to transport down to the planet to get a specimen, he attacks Scotty and a crewman. In the next scene when Kirk and Doctor McCoy show up, the crewman is holding his shoulder but receives no medical attention from Dr. McCoy through the entire scene.

After the Enterprise crew established a zero-tolerance viral quarantine protocol in Miri (1966), their lack of concern about bringing the infected Denevan colonists onto the ship is a bit odd.

As Spock struggles with bridge personnel to take over the ship, Yeoman Zahra is briefly seen smiling at them.