Revealing mistake: At Freddie Filmore's radio show, when Lucy steps out from behind the curtain, a bit of the kitchen from the Ricardo apartment is visible behind Lucy; it's the cabinet area with plates and canisters, beside the refrigerator. (00:06:15 - 00:19:40)
Continuity mistake: The step up into the kitchen is no longer there in later shows such as "The Fur Coat" and others.
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.
raywest ★
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.