In Star Trek VI: The Undiscovered Country (1991), the Excelsior has an LED clock above its view screen, but in this episode, it's missing.
Valtane reports that the energy wave is coming from the port (left) side, but it hits the Excelsior from the starboard (right) side. In addition, Sulu orders the helmsman to fire the starboard thrusters. This should turn the ship to the left, but the exterior shot shows the ship angling to the right.
The Doctor orders Kes to bombard Tuvok's brain with 20 kilodynes of thoron radiation. The dyne is a unit of force defined as "the force required to accelerate a mass of one gram at a rate of one centimeter per second squared", it is typically used to measure surface tension. Radiation is measured in either units of energy like the Curie or units of absorbed dose like the Rad or Gray.
Sulu states that igniting the gases in the nebula would be like "tossing a match into a pool of gasoline". In fact, this would only cause the match to go out. It is gasoline vapors which ignite, not the liquid.
Reused footage from Star Trek VI: The Undiscovered Country (1991) is mixed with footage of a newly constructed Excelsior model (the original model was converted into the Enterprise-B for Star Trek: Generations (1994) and couldn't be restored to its original state). The new model can be easily distinguished by the warp nacelles, which feature illuminated strips along their full length that the original Excelsior model lacked.
Tuvok repeatedly states that the explosion of the Klingon moon took place three days before the final battle scene; however, in the movie Star Trek VI: The Undiscovered Country (1991), the explosion of the moon is stated to have taken place two months before the next scene in the film.
When Sulu notices Janeway on the bridge, he calls for security after Tuvok does not answer whether or not he knows her. Meanwhile, Valtane's dead body is still lying on the floor, and Sulu nor anyone else bothers to call for a medical team.
Tuvok tells Janeway that he was 29 years old during the Khitomer incident; adding 80 years makes him 109 years old in the present. This is middle age for a Vulcan (Sarek, for example, was 103 in Journey to Babel (1967)), but Tuvok is obviously much younger than that.
No explanation is ever offered as to why Tuvok's service aboard the Excelsior does not appear on his Starfleet record. The issue is raised and then forgotten.
It is shown that Tuvok and Dimitri Valtane were bunk-mates, but a senior officer would not be sharing quarters with junior officers. At the time of the flashbacks Tuvok was fresh out of the Academy and carried the rank of Ensign, and it was common at the time for enlisted personnel and junior officers to share quarters. However Dimitri Valtane carried the rank of Lieutenant Commander, making him one of the ship's senior officers meaning he would have had his own private quarters.
Tuvok explains to Kes that the meditation he is performing is called Keethara, which he says translates as "Structure of Harmony." The Universal translator should make minor language lessons like this unnecessary. (Unless it's convenient to the plot or provides a cute character moment, in which case the show turns a blind spot to this matter.)
Sulu orders the helm to set a course for Qo'noS and engage. He never gives a speed.