Goofs

Lazarus' goatee changes several times.

In the second sickbay scene, when Lazarus is on the bed, the amount and shape of the blood around Lazarus' mouth changes in each shot, alternately getting larger, then smaller, then larger, then smaller again, all within the space of less than two minutes.

When Lazarus leaves his bed in the sickbay at around the 30 minute mark, the bed continues to broadcast his heartbeat. Indeed, when he starts to have an attack, his heartbeat speeds up. Normally, the sickbay bed can only read a person when he is lying on it; the sound should have stopped as soon as he stood up.

Whenever the two Lazaruses meet in the time corridor and possibly switch into each other's universes, there is always the "winking out" effect at that time as the time corridor is in use. However, when the non-bandaged Lazarus switches places with the bandaged Lazarus in sickbay (a switch not seen on-screen, but which occurred while Kirk and Spock were discussing Lazarus and the phenomena on the bridge), there was no such effect. In contrast, the effect did occur at all other times, including when Lazarus was alone in the Enterprise corridor.

After the fire starts in engineering and the assistant engineer drags Masters out of the room, when they are almost to the door, the smoke in the room is still relatively light and there is not enough of it to obscure vision. Less than 3 seconds later, when the door to the corridor opens, the smoke is so thick that it is not possible to see through it.

The main threat in the episode is flawed. While in reality matter and anti-matter coming into contact with each other WILL cause a tremendous explosion (both the matter and anti-matter convert instantly into energy with 100% efficiency) the particles do not need to be identical as stated. In fact, the instant that Alternate (i.e. anti-matter) Lazarus came into contact with ANY matter including atoms of gas in the air, the explosion would have occurred but while it would have destroyed the "Enterprise" and possibly the planet, it would have no effect on the rest of the universe and none on the anti-matter universe (although the matter Lazarus would have a similar impact there).

Kirk says that with the dilithium crystals drained, their orbit will begin to decay within 10 hours. Unless the Enterprise was actually within the atmosphere (where friction would slow down the Enterprise and cause its orbit to decay almost immediately, not 10 hours later), no energy is needed to maintain an orbit. The Enterprise would orbit indefinitely even if it had no power whatsoever.

As soon as Kirk entered the anti-matter universe, any part of his body, which of course was composed of matter, that came into contact with anti-matter, would have been converted to pure energy, destroying not only himself but also the anti-matter Lazarus and a large portion of the anti-matter planet they were on.

Lazarus never introduces himself by name on screen to Kirk, yet Kirk calls him by name during the first planet search. We come into Kirk's interrogation of Lazarus part way through, so it could have happened just before the scene begins.

Kirk wants "every inch" of the planet searched, but Starfleet is on the metric scale. However, this is merely a colloquialism that will probably persist.

After Kirk returns from the "Anti-matter universe", he approaches Lazarus from behind and grabs him in order to force him into his ship so that he will become trapped in the corridor that links the two universes. During this confrontation, the two security officers are clearly seen and are just standing there watching. Kirk glances over his shoulder and commands them to "Stay back!" but neither guard has moved at all.

In the end credits, Billy Vernon is listed as "SCPIPT" supervisor instead of "SCRIPT".

The plot heavily centers around engineering matters, including the theft of dilithium crystals as well as Lazarus starting fires in main engineering and attacking engineering crew members. With all of this, the absence of the Chief Engineer (Scotty) is extremely unusual (the real reason being that actor James Doohan was only contracted for a certain number of shows and was omitted from the script).

Kirk tells Lazarus, "Let's have it. The truth this time." His lips do not move during the second sentence.

Even after Spock has proved Lazarus to have lied, Kirk allows Lazarus to roam unguarded aboard the ship.

At the same time that Lieutenant Masters is attacked, Uhura reports to Kirk that Security alerted her that Lazarus was missing. However, Lazarus was not in custody, and had been allowed to wander throughout the ship, so he would not have been under Security's surveillance in the first place, and therefore could not be "missing".

After scanning the planet, Spock declares that it contains no life. The subsequent appearance of a singular humanoid caused Kirk, Spock, and a security team to beam down to the planet to investigate. They do find a humanoid; but there are also trees, bushes, and grass on the planet.

Where did these plants forms of life come from, and why didn't the ship's sensors detect them? Moreover, why isn't Spock or anyone else surprised by their existence?

The bold plan to trap both Lazarus's in the universe corridor is actually completely unnecessary. Once Kirk learns the truth, he could simply take the insane Lazarus into custody, destroy the ship, and leave the planet. The antimatter Lazarus would never build another ship and the insane Lazarus would either be imprisoned or possibly killed by Starfleet to safeguard the universe.

Another episode in which we see an alien successfully use the transporter when they have no knowledge how to do so. Lazarus does this when he escapes with the crystals. Although story wise, this is done simply to keep the flow events going, in reality, any number of catastrophes could happen. He could wind up scattering his atoms throughout space, he may not re materialize at all, he could re materialize disfigured, and this is just to name a very tame few.

At the very beginning when Spock is reporting the atmospheric composition of the planet he has scanned he says "Oxygen Hydrogen atmosphere". This would be a very explosive combination. He meant to say "Oxygen Nitrogen atmosphere" (similar to Earth) but just got his line wrong.

Charlene Masters is addressed as "Lieutenant" several times, but her tunic has the braidless sleeves of an Ensign. Moreover, she wears a blue sciences tunic, but she works in engineering, which involves ship's services (red tunic) personnel.

When the ship goes to Red Alert, both Kirk and Spock leave the bridge to investigate the sign of human life on the planet, and nobody takes command. This happens several times. Under Red Alert, Kirk usually orders another officer of command rank to take command of the ship. This officer would normally assume the captain's chair.

Charlene Masters is addressed as "Lieutenant", but her braidless sleeves indicate she is an ensign.

The Enterprise crew, and Starfleet, refer to a portion of the galaxy as 'quadrant', and later, as 'Section'. It is likely they meant 'sector', which is a lot smaller than a quadrant (one fourth of the galaxy). Apparently the quadrant system of Star Trek: The Next Generation (1987) onward had not yet been determined by the writers.