Deliberate mistake: It's not particularly believable that the away team to stop the assassination would consist of the ship's entire senior staff, and raises the question of who was left in charge of the bridge (the answer: apparently nobody, as they return to a completely deserted bridge). Of course this is dramatic license, and at this point the audience won't care or likely even notice.
Other mistake: Throughout the film, Valeris is referred to as a Starfleet Lieutenant. However, in every scene, she wears the rank of Commander on her uniform. Best comparison can be made in Star Trek II: The Wrath of Khan where the same uniforms were used. Saavik (Kirstie Alley) wore a Lieutenant rank (2 piece), where Chekov and Scotty (himself then still a Commander, until promoted by Admiral Morrow in Star Trek III) wore the single-piece rank insignia used by Valeris in The Undiscovered Country.
Continuity mistake: It's slight, but noticeable. The nails used to keep Chang's eye patch in place change slightly over the course of the film. For example, when they're beamed aboard the Enterprise, the nail closest to Chang's eye is at the top of the patch. But, during the climactic battle, it's down, closer to his nose.
Other mistake: It was shown that whatever form Martia was in, whether it be the attractive semi-human form, the Sasquatch form, or the little girl form, her voice always remained the same. Why didn't her voice remain when she morphed into Kirk?
Suggested correction: Clearly she has the ability to control when and how her voice changes in the same way she controls her physical form.
Continuity mistake: In the original UK VHS version of the movie, Scotty beams the Klingons aboard the Enterprise using the controls in the booth (located behind a glass screen). The shot cuts from the booth interior to the Enterprise senior officers awaiting the arrival of the Klingons on the transporter pad - Scotty is suddenly in front of the glass and in the line-up. The shot cuts back to the interior of the booth and Scotty is back in there, operating the controls.
Revealing mistake: When the Enterprise begins to leave the spacedock, the background inside spacedock (behind Enterprise) seems to move in two different directions, the top moves towards the right, the bottom towards the left.
Continuity mistake: At the dinner you can see the contents in Kirk's glass changing from full, to nearly empty, to half-full again, without Kirk either drinking from the glass or having the glass refilled.
Other mistake: Every time we see a high ranking Starfleet Officer in the Starfleet Command scenes, they are wearing dozens of different badges and medals. Why don't Kirk and crew have any? In just the movies, Kirk and Co. have saved Earth twice and foiled an internal assassination attempt against the Federation President, as well as all the heroic and successful missions carried out in the original series. In the classic episode "Court-Martial" it lists Kirk as receiving the Starfleet Medal of Honor, the Silver Palm with Cluster, Starfleet Citation of Conspicuous Gallantry and Kragite Order of Heroism. And it is appropriate to Starfleet regulations to wear them. Kirk was wearing two decorations in the TOS episode "Court Martial" when he was framed for a crewmans' death.
Character mistake: There's no apparent reason for Spock's conviction that the gravity boots are a damning piece of evidence that will reveal the assassins, especially considering how useless they prove to be after being found planted in an innocent man's locker.
Character mistake: At the trial, the witness said that "With the first shot, we lost our gravitational field. I found myself weightless, and unable to function." However, the first shot caused the ship to start listing to port. It was the second shot that caused the gravitational failure.
Suggested correction: People misremember events all the time, especially if the events are unexpected and chaotic. At best a character mistake.
I called it a character mistake.
He could also simply be lying in order to explain why he did not do whatever he should have after the 1st shot, since he was an easy target for the intruders and did not die in battle, which would have been honorable. Klingons are not the most honest of people.
Continuity mistake: When the Excelsior enters the battle in orbit of Khitomer, General Chang's Bird Of Prey is on top of her. However, when he orders to fire upon her, she is hit from below.
Continuity mistake: At the end, as the crew are talking about being arrested for feelings, the time on the bridge clock is 17:56:19 as Chekov sits down. We hear the viewscreen activate, cut to see Sulu talking, and the time above the screen is suddenly 17:56:28.
Continuity mistake: Valeris seems to appear out of nowhere during the thwarted assassination attempt on Khitomer. The Enterprise crew beam down without her, spring into action, and when Admiral Cartwright yells "Arrest those men!" Spock emerges from the crowd with Valeris in his custody as if she were always there with him or he'd pulled her out of a hat.
Character mistake: Chekov pulls a gun on the Klingon ambassador even though he hasn't been implicated in the conspiracy.
