Character mistake: Spock says that Vanderberg said there were thousands of these silicon nodules, but Vanderberg actually said there were millions of them. Not a mistake Spock would make.
Character mistake: When Spock and Kirk pursue the Horta, they come to a fork in the tunnel. Kirk points right and tells Spock to go left. Then he points left and says he'll go right.
Character mistake: Kirk informs Spock via communicator the Horta is ten feet from him, and Spock insists Kirk kill it. First, both know they cannot kill it with their phasers, and second, Spock's demand for Kirk to kill the Horta runs counter to the Vulcan philosophy of respect for all life. Spock would never want to harm, let alone kill, another life form.
Character mistake: Shatner fluffs his line when asking the mining engineers whether they've posted sentries. He asks if they've posted "centuries" instead. (00:05:20)
Suggested correction: At this point they don't have any proof that they can't kill it, and since Kirk is in danger, it's logical to try. Also, Spock is half human and he's concerned about his best friend being killed. Logical or not, he'll want Kirk to protect himself at all costs. Other episodes have shown where Spock doesn't always behave logically when his friends are at risk and he lets his emotions come out.
envisaged0ne