Hollywood Anniversary - S4-E24
Continuity mistake: In season 2, "The Anniversary Present," Lucy has the 19th circled on the wall calendar, which is the date of their wedding anniversary, but in this episode when the license bureau in Greenwich, Connecticut sends Ricky a telegram with their wedding date, it's now on the 7th.
Answer: According to Snopes.com, there is no definitive answer, but the mid-1960s is the most verifiable date with "The Munsters" being cited as the first, although others claim "The Brady Bunch" showed the first couple seen in a double bed. An early TV show from the late 1940s titled, "Mary Kay and Johnny" is also thought to have shown the married couple's bedroom as having a double bed, although probably not with them in it. However, this was when TV was aired live, and there are no surviving episodes, only anecdotal accounts.
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Something that is funny is that in the movie "A Christmas Story," they show the parents having two twin beds in their bedroom. In a real situation, they should have shown them having a double bed. Lucy and Ricky had twin beds pushed together in an early episode, which would have been pushing television boundaries in that time.