Continuity: Just after the ship is first attacked, Scotty tells McCoy, "We can't fire full phasers with our screens up." Say what? The Enterprise couldn't operate transporters with the shields up, but it fired its phasers, full or otherwise, with the screens up every time it went into battle, and always had. If they'd been forced to shut their shields down every time they fired, the Enterprise would have been history long before this.

Factual error: How do the Eminiarians fire a weapon based on sound ("Decibels - 18 to the 12th power") up into the vacuum of space at the Enterprise?

Continuity: When Spock and Kirk pursue the Horta, they come to a fork in the tunnel. Kirk points right and tells Spock to go left. Then he points left and says he'll go right.

Continuity: After his initial mind meld with the horta, Spock tells Kirk, "That's all I got, Captain: waves and waves of searing pain." A minute later, he says that it's "a highly intelligent, extremely sophisticated animal" that calls itself a horta. Apparently, waves of searing pain were not all that he got after all. Unlike Spock to be so imprecise.

Plot hole: Spock has absolutely no way to know, yet, that the horta only secretes her corrosive substance when tunneling: he hasn't had time to examine her or to do more than determine that she does indeed secrete a substance that cuts the tunnels. So he should at the very least scan the piece of her that falls off before he picks it up with his bare hands. Major lapse of logic, which given his character goes beyond a character mistake.

Deliberate "mistake": Here, as in nearly every other ST episode featuring caves, caverns or mines, all the floors are perfectly flat and the lighting adequate-to-see-by or even brilliant. These configurations are easily found on soundstage sets - but not in nature.

Revealing: The Horta's round tunnels have perfectly even striations and are obviously factory-manufactured tubes. Acid secreted by an elliptical creature burning through solid rock would not create a perfect circle. The Horta is visibly not chewing, sculpting or smoothing the sides. No acid burning method would leave patterned stripes on the walls, either.

Revealing: Shatner fluffs his line when asking the mining engineers whether they've posted sentries. He asks if they've posted "centuries" instead.

Continuity: When Kirk orders the crew to fire phasers; the exterior shots show the ship firing torpedoes, and torpedo explosions flashing out in space.

Visible crew/equipment: When Kirk is thrown into jail with Spock you can see the mat used to break his fall.

Continuity: The Organians cause all the weapons to overheat, and everyone drops them to the floor. When Kor moves to the desk to call his ship, though, he's wearing his disruptor. It's gone again when he returns.

Plot hole: Kirk knows that Lazarus is insane and that he wants the Enterprise dilithium crystals. Yet he's not restrained in sickbay and is, in fact, given free run of the ship so that he can knock out the crew in engineering and steal the crystals. Other than to further a woefully weak plotline, this makes no sense whatsoever.

Continuity: Both the positive and negative versions of Lazarus have a long mustache and a fairly full "waterfall" beard - until the scene in the briefing room, when inexplicably, the mustache and beard are both suddenly so thin they're barely visible. As soon as they beam down to the planet, however, the much thicker facial hair is restored.

Continuity: Mad Lazarus has just put a black-and-white hatch cover over the stolen dilithium crystal in his time ship. It's in place when Kirk arrives to confront him. But a moment later, when Kirk leans in and is accidentally transported to the negative universe, the cover has disappeared. It's still missing when Kirk returns to push Lazarus through the portal.

Continuity: Kirk keeps referring to Masters as "Lieutenant", even though she has no braid on her sleeve - all Enterprise lieutenants have a single gold braid on each sleeve.

Factual error: Edith Keeler says to McCoy, "My young man is taking me to see a Clark Gable movie." In 1930, Clark Gable was an uncredited bit player who had last made a film in 1926 - Edith, a New York charity worker, would not even have encountered his name.

Continuity: After Edith stumbles and nearly falls down the stairs, she still has both her shoes on. But after the kiss, she proceeds up the stairs and is now carrying one shoe. She had no time to take it off, as she was, er, busy with Kirk the whole time.

Continuity: As Kirk and Spock prepare to enter the time portal, Spock deliberately places the tricorder at his left side, holding it there with his left hand. As they appear in 1930 earth, he is holding the tricorder with his right hand and his left hand is free.

Continuity: While he and Kirk discuss the history tapes in their room, Spock's shirt collar is turned up on the left side in close-ups, but straightened out in the full shots.

Continuity: When they put the satellites in orbit, Sulu announces they are in place. Then Kirk says "Energize" and the next shot shows the helmsman station and Sulu is absent - some guy in a red shirt is there instead.