So the script has McCoy addressing Lt. Rodriguez by his last name, then as communications break down, by his first name. No error in line delivery on De's part.
In the episode with the Mugato, it was originally "Gumatu", but De Kelley kept fumbling the translation and they ended up changing the word to "Mugato" to help him out.
Barbara Baldavin played Angela Martine (marrying Tomlinson, who was killed off before they were pronounced man and wife) in "Balance of Terror", and returned to play a different character, Mary Teller, in "Shore Leave". She suggested she could still be Angela and the director agreed it was a good idea, and Teller was changed to "Angela Martine" in the scripts. However, one scene had already been filmed with Shatner saying "Teller", so she ends up being Angela Teller. In the final episode, she returns again, and Sandra Smith, playing Kirk, mistakenly calls her "Lisa". (Her brief scene as Angela Baker was cut from "Space Seed".)