I thought the very strangest casting choices for guest stars were the ones where a recognizable guest star repeated in a very different role. Mark Lenard's role in "Balance of Terror" was not small or easily-overlooked, so seeing him again as Spock's father in "Journey to Babel" was rather a shock. And subsequent watchings of "Balance of Terror" -- after seeing him as Spock's father -- were pretty surreal. Surely Spock's father wouldn't attack the Enterprise
? I know they're estranged and all, but isn't that taking things a little too far?
Almost as surreal was seeing Trelane show up as a Klingon captain in "The Trouble With Tribbles." WTF? How did Trelane become a Klingon? Again, Trelane was not a small role, so we couldn't fail to recognize his face when he appeared again in "The Trouble With Tribbles."
I guess things were different in the 60's.