When Data informs Picard that the energy residues might indicate that the Romulans are involved, Picard notes that they haven't heard about the Romulans in "a while". Yet in Angel One (1988), a potential battle with the Romulans is one of the secondary plot points.
When Lieutenant Yar arrives at the detention cell, she finds her security man down, and Konmel dead. While still crouched over her man, she reports this to the bridge, and concludes, "Korris has escaped and is armed." She hasn't looked in the detention cell to see if there is anyone there, alive or dead, and if the second security man was still armed. As such, Tasha has no way of knowing that Korris has escaped or that he is armed.
When Yar and her security team attempts to take Korris and Konmel into custody on Deck 17, the turbolift in the background puts them on Deck 21.
When Riker, La Forge, and Data are walking through the corridor on the Talarian vessel they change positions between shot although there are not actual time lapses.
Korris is alternately referred to as a commander and a captain. Since he is not in command of a vessel, this couldn't be a case of a ship's commanding officer being addressed by the honorary title of captain, regardless of rank.
When in Engineering, Yar tells Picard Korris has a phaser/disruptor aimed at the dilithium crystal chamber (Worf incorrectly says the same in the Observation Lounge). He is targeting the warp core/core casing.
When Worf blasts the Klingon in the warp core, he falls into the floor that shatters like plate glass. Even in the present time, there are polycarbonate transparencies that do not break in that manner.
Although Konmel is an officer (a lieutenant), he wears the uniform of a Klingon enlisted man: his vest is closed with no v pattern to it, and his belt buckle is without a gold frame around it.
Worf states that he has detected signs of a battle in quadrant 9 of the Neutral Zone. A quadrant, by definition, is one-fourth of something. There cannot be any more than 4 quadrants. However, this misuse of the word is not uncommon in space operas or military fiction and is just an imprecise usage, in the same way as a city can have more than four 'quarters' (like the Latin Quarter in Paris).
When Picard calls Korris's guest quarters to inform him that his comrade in sickbay has taken a turn for the worse, Korris is able to reply to Picard simply by speaking aloud. Since the Klingons do not have commbadges, they should have needed to use an in-wall computer terminal to communicate. However, it is clear that the comm system can pick up voices from a distance, as in the frequent occasions when officers are called when they are bed, not wearing comm badges.
Several members of Yar's security team do not have rank insignia on their uniforms.
The dual barrels of the weapon the Klingons assemble in the brig are obviously plastic and barely stay together.
When Worf and Picard first enter sickbay to greet the Klingon survivors, their overdubbed dialogue does not match their mouth movements.
When the Klingon vessel is within range, we hear Picard say "Open frequencies" but his lips say something else just before the word 'frequencies'.
When Korris is in engineering, aiming a weapon at the warp core, no-one suggests the obvious solution of locking onto him with the transporter and beaming him away from engineering.
When Worf addresses Captain K'Nera on the bridge, a mic can been seen in the top of the screen.
When Picard goes to sickbay with Worf to check on the Klingons after their arrival, he doesn't leave anyone in charge of the bridge, as he normally would - particularly since none of the other senior staff are left on the bridge either.
When Korris is in engineering, Worf continually calls the weapon that Korris is holding a phaser. The Klingon weapon equivalent to a phaser is called a disruptor.
When asked if he'd spent much time among Klingons, Worf replies, "Hardly none." "Hardly none" would mean a great deal, which is not what he wishes to convey. He should have said "hardly any" or "almost none."