Continuity mistake: Kirk and Spock pretend to be filming Daras so they can enter the building where John Gill will give his speech. Look at the lamp Spock is holding. In the beginning it works, than a couple of scenes later it doesn't, and when they are outside the chamber where John Gill is, his lamp is on again.
Continuity mistake: After his initial mind meld with the horta, Spock tells Kirk, "That's all I got, Captain: waves and waves of searing pain." A minute later, he says that it's "a highly intelligent, extremely sophisticated animal" that calls itself a horta. Apparently, waves of searing pain were not all that he got after all. Unlike Spock to be so imprecise.
Continuity mistake: Towards the end of the episode when the Enterprise is moving leaving from the starbase, the ship is at an angle to the camera pointing to the right of the screen. The starbase behind it gets smaller and goes up a little. If the Enterprise were leaving the starbase at that angle, the starbase should have moved to the left and then completely off the screen.