As Kirk orders the landing party to split up to find water, there is a close-up of Dr. McCoy with a strap over his left shoulder, but when the view switches to Kirk and McCoy the strap is missing.
The first time Losira appears for Kirk, his right hand is on a dial on the tricorder. In the next shot his right hand is at his side, then on the following shot it is back again to the tricorder.
At warp 8.4, traveling 990.7 light years would take 1.67 years, not 11.33 hours (which would equate to 766,486c or warp 9.15).
Enterprise's navigational sensors would have alerted Lts Rhada and Hadley that they had been sent 990 light years across the galaxy. They would not have determined it by looking at the viewscreen.
Sulu makes reference to the Hortas of The Devil in the Dark (1967), but Sulu did not appear in that adventure. However, he was still a senior crew member and would have heard or read the reports.
During the opening "earthquake/storm" scene (as soon as the landing party beams down), when the lightning flashes, distinct shadows can be seen on the "sky" behind the rocks, exposing that it is a fabric backdrop/screen.
All the controls on the Enterprise are just buttons and lights and nothing is marked by function. One would assume that even an analog gauge would be appropriate for some operations. Lights and unmarked buttons would be impossible for people to remember their function.
When Sulu fires at the ground to stop Losira coming any closer there is a very loud explosion which fails to alert Kirk and McCoy who are asleep close by. Yet, when Sulu then calls out to them they awake and come to his rescue.
Initially, the purpose for beaming down a landing party is purely scientific, to investigate a geologically odd planet. Kirk's decision to send Sulu instead of Spock on this mission makes no sense.
The Enterprise was thrown 990.7 light-years from the planet they were investigating, then return in about 1 day. However 7 whole seasons of Star Trek: Voyager (1995) were based on the idea that it will take 70 years to travel 70,000 light-years. With the rate stated in this episode, Voyager should've passed that distance in little over a month.
When crewman Watkins is confronted by the alien, Losira, in engineering, he should have reported her immediately, but he talks to her and even discusses operation of some of the engineering controls. This would be a breach of every military procedure.
Mr. Spock is always written to have a precise command of language, yet when Scotty reports that the ship "feels wrong", Spock seems unable to distinguish the difference between "feeling" as an emotion and "feeling" as a physical sensation along the same lines as "seeing", "hearing", etc.
Left in charge of the ship, Mr. Spock uncharacteristically browbeats everyone on board just for being human (seemingly not free of emotions as he's always claimed). Given that he's the science officer and that the Enterprise orbits a planet whose very existence is inexplicable, it's reasonable to hypothesize that he's getting back at Kirk for leaving him out of the away mission by belittling the crew at every turn, thereby becoming a character ruled by the emotion of bitterness.