Continuity mistake: As there were two screen used models of the Enterprise, when the ship is seen flipping from one universe to the other and back, the nacelles of the Imperial Enterprise have the solid red fronts with what appear to be antennae.
Continuity mistake: As Kirk and party dematerialize, Kirk's arms are by his side. As they are briefly seen in the Transporter room, his left hand is up with the communicator.
Revealing mistake: In the close up shots of the mirror Sulu, the poor make up job of the scar on his face is very evident.
Continuity mistake: At the end, Lt. Moreau gives Kirk a report to sign. After signing and giving it back, and after a dialog with Spock, he gets up, goes to Moreau, has the report and looks at it again.
Continuity mistake: Throughout the entire episode the planet's sky is red, yet during the scenes where the clouds come and lightning strikes, it becomes obvious that they simply used stock footage of Earth's clouds, since the sky is blue. (00:11:50)
Visible crew/equipment: When Vaal is not fed and he starts spewing smoke everywhere, you can see a volcano type firework going off just inside his mouth. (00:43:55)
Continuity mistake: When Kirk gives the order to Scott for transport, Spock lies in a diagonal position with the yellow spores on his chest with McCoy kneeling by him. After the cut to Scott on the bridge, Spock is lying with his head towards the camera, his chest is clean, and McCoy standing about six feet to Kirk's right. All much too quickly to be normal.
Visible crew/equipment: On the Constellation, the camera is looking at the man helping Kirk to get the secondary view screen working. As the camera goes to pull back to allow Kirk into the shot, you can see the shadows of the camera shades on the wall. (00:25:00)
Revealing mistake: While Kirk watches, the machine fires at the Enterprise. As Spock is knocked backwards, watch his hands as he goes forward. They automatically go into a certain, almost practiced position. Then a moment later, the Communications officer is reporting to Spock. As she does, another shot hits the ship. She and just about all the crew react to the jolt a second before the camera and lighting crews simulate the hit. (00:26:40)
Continuity mistake: First, Scotty rigged a self destruct delay timer for Kirk that supposedly had thirty seconds programmed in. However, putting a timer against the one used in the episode, from start to finish there was one minute twenty seconds before the Constellation blew up. Second, Kirk was holding the communicator out and away from himself before his beaming, then it's close to his face as he materializes. (00:47:05)
Continuity mistake: As Decker is talking about the Planet Killer, the camera looks at McCoy. His left eyebrow is slightly arched, and his hair is styled differently. It can be seen that this shot is reversed from normal.
Audio problem: About a minute or two into the episode, Captain Kirk says "That's incredible" but his mouth doesn't match the words.
Other mistake: When the Planet Killer hits the Enterprise, the ship lurches 45° to starboard, and the crew fall accordingly. However, the chairs, except for the communications station, stay in place.
Plot hole: Commodore Decker takes over command of the enterprise but Spock says if McCoy can certify him incompetent (which he obviously is) he can be relieved of command. McCoy says he will certify him now, so why doesn't he? In other episodes it has been stated that McCoy can order anybody regardless of rank to an examination to see if they are physically or mentally fit.
Continuity mistake: In most shots, the Constellation's impulse engines are lit up orange, but when moving to meet the Enterprise, they're not lit up, though the ship is moving.
Continuity mistake: Spock nears the command chair, where a seated Decker is leaning on the left armrest. A second later he is leaning to the right.
Continuity mistake: Decker is leaning over the forward console between Sulu and the other man, with Spock at his left elbow. As he gives the order to veer off, he starts to turn to his right. As the angle changes, it appears that he was standing straight up the whole time.
