I remember Mark Goddard, one of the actors in LIS, recalled how he couldn't stop laughing because an episode he was so stupid he couldn't keep a straight face...even the final cut has him smiling.
You're probably thinking of 'The Great Vegetable Rebellion,' Lost In Space
's next-to-last episode. It guest-starred Stanley Adams, who appeared as Cyrano Jones in Star Trek
's 'The Trouble With Tribbles.' In TGVR, he is Tybo, the carrot man leader of the Vegetable Planet. So utterly ridiculous it's hilarious, especially when he turns Dr. Smith into a celery stalk.