I remember this from a GSN rerun, there was a 70s-era \"Family Feud\" where a VERY old man was one of the contestants, and Richard led off by saying something like \"Let us know if you\'re okay,\" which I thought was an odd thing to say. Late in the game, Richard comes to the old man and he has his head down, resting on his hand. Richard looks at him, turns to the folks off-stage, and yells \"Don\'t sound the buzzer! This isn\'t a strike!\" and walks right past him and goes to the next contestant in line.
That\'s the one that popped into my head when I read the first post. I can\'t think of any other mid-episode occurrences; I do know about the ABC Dream House with the couple that couldn\'t return as the wife went into labor between shows, and I read about an episode of 1950s Name That Tune where a contestant wouldn\'t come back because she had to sing in her church choir.
Speaking of Feud, what about the episode where Richard had to be replaced in the middle of the episode?