Goofs

Just before Chekov forces a kiss on Mara, we see, in close-up, that she is wearing lipstick. Kirk and Spock then arrive and intervene, and, in the cutaway shots of Mara during Kirk's subsequent speech, the lipstick is gone. Then Kirk has Spock take her, and, as she and Spock exit frame, she's wearing lipstick again.

As Kirk, Spock & Chekov arrive on the bridge from the turbo lift just after McCoy got off on Sickbay deck, Chekov is on the left side of the turbo lift as the door opens. However when the camera pans showing the 3 men getting out of the turbo lift, Chekov is seen leaving from the right.

While the Klingons are held in the crew lounge, the swords first appear and Kirk's crew falls back, fighting into the corridor. Attached to the wall In front of the turbo-lift is a bell-shaped prop, propped in place by an avocado-colored stand. You can see it gets kicked by a couple of red shirts. When Kirk and crew turn to run towards the lift, the prop is now on the floor. This can be seen at approximately 15 minutes into the show. Then, at about 19 minutes, when Chekov exits the turbo lift at the same location as the previous sword battle, the bell-shaped prop is back against the wall on the stand as it was before.

When the ship accelerates to Warp 9 and Sulu tells Kirk the new heading, Kirk says, "That'll take us out of the galaxy." Any constant heading would take them out of the galaxy eventually, although several headings (especially out of galactic plane) would do so quicker than others.

When Kirk pulls Chekov from Mara and puts him against a wall Chekov runs his palms against the wall and smears on it Mara's bronzing makeup.

Spock tells Kirk that he will have 7.9 seconds to get to the pad after he energizes the transporter. With no sense of urgency, Kirk takes his sweet time and doesn't bother to get to the pad for about 12 seconds, with a few seconds to spare. (If Spock was that far off in that computation, it's a miracle he got the beaming coordinates correct to the extreme precision they make clear is necessary.)

In the last scene when Kirk and Kang are fighting with their swords, no sound effects are used and you can hear that they are plastic props.

In the episode teaser, Walter Koenig (as Chekov) had forgotten that he was supposed to be taking tricorder readings immediately upon beam down. When Kirk asks for a report he quickly grabs the tricorder and begins scanning.

When the Entity begins to take over the Enterprise by increasing its speed and taking it on a course out of the galaxy, the scene begins to shake suggesting it is a rough ride for the crew. As the scene changes from the bridge to Engineering, Scotty is explaining to Captain Kirk that the engines have "Gone to Warp 9, by themselves". As he is saying these words, the scene suddenly stops shaking. There are no other indications that the ride has suddenly become smooth. The camera operators and/or the special effects team are completely out of sync.

As Kirk is giving instructions to Spock to scan for other ships, the orders are obviously and awkwardly spliced in from looping.

After Bones storms off the bridge there is a reaction shot of Kirk where you can see smoke drift in from the left side of the frame.

After Scotty finds the claymore (Scottish longsword), Kirk and Spock are on the bridge, using Spock's scanner to locate where both the ship's crew and the Klingons are located. On the left side of the frame, you can see the wooden edge to the set.

When the Klingon survivors are beamed from Kang's battle cruiser to the Enterprise, none of them show any injuries. This after multiple explosions on board have killed all but some forty of Kang's crew.

Chekov attacks the Klingon escorting Mara in the corridor, and he falls to the floor, behind where Chekov grabs Mara. However, there is no sign of him on the floor when Spock and Kirk arrive and Kirk pulls Chekov away from Mara, knocking him out. Moreover, neither Kirk nor Spock give any sign that they are aware of the prostrate Klingon lying literally at their feet when they enter the corridor, or at any other time in the scene.

On the planet's surface, Kang tells Kirk that the Klingons have been at peace with the Federation for three years, and that they have honored the terms of the Organian Peace Treaty to the letter. What he's unaware of, or chooses to forget (as does Kirk), is the intrusion into Federation space and unprovoked attack on the Enterprise by a Klingon battle cruiser in the earlier season 3 episode: "Elaan of Troyius".

Immediately after The Entity takes over the ship, and Kirk states, "That'll take us out of the galaxy", the camera snaps to Uhura who begins to tell Kirk about the reports that are coming in from the lower decks. She is speaking directly to the camera for several seconds, as if she is making a report to be seen by Kirk on a view screen. Then Kirk steps up to her from the right side, clearly startling Uhura, who then finishes her report to him.

When Scotty and Sulu subdue the Klingons just outside of Engineering, Scotty says, "They've taken engineering," but the fight is still raging behind the door. Scotty, Sulu, and the "red shirt" would have gone back in and rejoined the fight.

Kirk shows very poor decision-making when beaming down to the planet, into a potentially life-threatening situation, by taking only himself, Chekov, McCoy, and one red shirt, instead a squad of fully armed red shirts.

In another show of questionable leadership abilities, Kirk freely informs Kang about the dangerous situation that the Enterprise is experiencing: the out of control speeds of the ship, heading off course from Starfleet toward the edge of galaxy, and most importantly, that the bulk of the Enterprise's crew is trapped and cut off from the rest of ship. Potentially dangerous information to give an enemy under any circumstances.