Yes, that was in the book "The World of Star Trek". Takei was in the middle of doing "The Green Berets" and when he heard about "The Gamesters of Triskelion" he tried to get the ok from the producers to return to Desilu (or was it Paramount by then?) to film the scenes for "The Gamesters...". I think he had figured out that he had enough time to go back to do the episode and get back to "The Green Berets" in time to film his next scenes. However there were delays, probably due to the weather delays I would imagine, and it ended up being too late to get back to do the episode. He was very disappointed, he really wanted to do the episode and if it weren't for the delays he probably would have been able to do it. Not sure about "The Trouble with Tribbles" though, I don't recall seeing that. One benefit of Takei's absence is it did give Koenig time to settle into being a (almost) full time cast member, since he had to sort of fill in for Sulu (usually the once named Lt. Hadley was at Sulu's station during those episodes but unfortunately was never given any speaking roles--he's got to be one of the most seen, never heard from characters in all of Star Trek--he was almost on enough to get a billing in the end credits, yet he never spoke a single line; in Turnabout Intruder they should have given him one line almost as a joke, even a "yes, sir" line--fans would have been like Holy S---, HE SPOKE).