Goofs

While transporting Kirk back onto the Enterprise there is a close-up of the transporter operator's hands. His rank has changed and he is missing a finger on the right hand - clearly those are Scotty's hands and rank.

Scotty has his hair cut short and combed down in this episode. But, in one scene, where Spock calls him right before inter-phase, the scene shows his hair combed back, in the style he wore in the first few episodes of the third season. That shot was re-used from The Paradise Syndrome (1968).

Chekov becomes hysterical on the bridge and two security guards drag him to the turbolift in order to restrain him in sickbay. Two minutes later, a Tholian appears on the viewscreen and Chekov is seen in his usual chair. This is due to reuse of stock footage.

When the landing party first beams aboard the Defiant, the shot is flopped to maintain continuity of Kirk's line of sight. Kirk's hair is parted on the right side of his head.

The first shot of Chekov in the Defiant's engineering room shows him bobbing back and forth unnaturally, because the film was rolled backward and forward a few times as filler.

It's obviously a stand-in for Scott, and not James Doohan, whom the mad crewman jumps down upon in the Engine Room.

Obvious stunt double for Mr. Scott saying "Scott here" when Spock calls the engine room.

After the Enterprise disables the first Tholian ship, at least one star can be seen through the drifting ship.

Chekov screams though his mouth does not move.

When Chekov and McCoy return to the Defiant's bridge, via the turbo lift, the turbo lift doors open with a distinct "clunk". However when the turbo lift doors close, they close with the usual "swoosh" sound effect.

When the crew first encounters the Defiant, Spock makes the point several times that, although the crew can see the ship, the Enterprise's sensors cannot detect it, so, according to the computer, the Defiant is not there. Sulu has trouble properly maintaining the Enterprise's distance from the Defiant because the instruments are misleading them. Yet the transporter chief seemingly had no problem locking onto the Defiant's bridge and transporting the away team there.

This is the second episode in which McCoy has berated Spock about taking command. The first was in Season One's episode The Galileo Seven, in which McCoy assumes that Spock is taking great pleasure in being in charge of the desperate situation they are in. Here though, it has been contrived solely to have a secondary conflict in the story. At no point throughout the series has Spock ever shown any ambition to command either the Enterprise or any situation for that matter. Spock completely embodies Starfleet rules and regulations, almost to the exclusion of all else. Spock is second in command. When the Captain is "unavailable," it is Spock's duty to replace him. It has been well established that McCoy enjoys bickering with Spock, but he still considers him a friend, and he would know that Spock is incapable of "enjoying" assuming command.

When the Defiant is seen on the main view screen, Spock reports that it isn't indicated by any of the sensors on the Enterprise. He says; "Our sensors indicate it is not there." However, this is wrong, as the inability to detect something does not prove its absence. Spock should have said; "Our sensors do not indicate anything there."