Revealing mistake: At the Brown Derby, when Lucy bumps into the waiter causing him to trip and spill the tray all over William Holden, we can see that the waiter's hand is taped to the bottom of the tray for the stunt to be done efficiently. (00:15:15)
Continuity mistake: At the start, when the Pontiac convertible turns into the driveway of what should be the Beverly Palms Hotel, the sign with the real name of the hotel can be seen, which reads Beverly Carlton Hotel.
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.