Mr. White's Christmas

In this penultimate episode of the series, a new member of the family, the hitherto-unmentioned cousin Mark Fleming, is abruptly introduced. Larry Hagman, who played him, explained in his autobiography that the series had fallen so far behind schedule that regular star Gig Young had to leave to take up a movie role, and so could not take part as intended. The script was hastily rewritten to include this new character, who also appears in the next and final segment.