Lucy Wants New Furniture - S2-E28
Continuity mistake: At the start of the episode, when Lucy puts the baby in the crib there's only one bumper pad and it's on Lucy and Ricky's side of the crib rails, but in the baby's closeup there's a bumper pad at the rails behind him - which is the opposite side of the crib. (00:00:45)
Lucy Wants New Furniture - S2-E28
Continuity mistake: After Lucy mentions the ad for a couch and coffee table, when she walks over to the desk note the shadow and lighting change on the back of the desk. (00:02:10)
Lucy Wants New Furniture - S2-E28
Continuity mistake: Lucy runs back and forth from Ricardo's and Mertz's apartment through a door in Mertz's living room that is not there in other episodes before or since.
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.