Several times, Scotty is drawn with a captain's extra braid on his sleeves.
Close-ups of Scotty operating the transporter clearly show a mustache sticking out from his face. This is actually footage of Mr. Kyle, the other transporter operator.
Due to the use of stock footage, the bridge is infested with tribbles in one scene and tribble-free in the next.
When Kirk talks to Koloth on the viewscreen, Koloth's entire body suddenly appears to be on the bridge of the Enterprise in front of the viewscreen.
When the Klingon weapon fails (trying to stop three ships at once), a subsequent bridge scene has the blinking lights on Uhura's console briefly appear *over* her face.
In The Trouble with Tribbles (1967), it is established that tribbles let out an ear-piercing screech when close to a Klingon; however, when beamed aboard the Klingon ship in this episode, they continue to purr happily.