Gamesters of Triskelion, Shatner flubs a line and it went through:
"What's happening to Lt. You-Hear-Ra?"
Pretty much anything Yeoman Mears says in Galileo Seven makes me cringe ("Oh...it's getting hotttt!" "We could use a little inspiration!" "I got a little bump on the head." Geeeeez).
And having Yeoman Zahra point out the aliens in Operation--Annihilate! don't look real just reinforced that they, well, didn't look real.
Jonas Grumby wrote:
And in "Balance of Terror," in at least one scene, Kelley (again) pronounces the name of the commander of the Fesarius, Balok, as something like "Baal-lik," while everyone else pronounces it "Bay-lock."
Corbomite Maneuver, actually.