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.
I have long heard (don't ask me for a source -- probably an old issue of Starlog, or just fannish gossip) that Lenard was approached to portray Abraham Lincoln in The Savage Curtain
but was unavailable, so the part went to Lee Bergere.
Can anyone confirm?