Episode #1.5 - S1-E5
Other mistake: An enraged Richard tries to get the drawing board in position making a bit of a ruckus. If you look at the board, it has names and pictures on it; they are from the future, since they are part of the 6th episode, "An Unhelpful Aid", evidently filmed earlier, out of order. (00:07:25)
Answer: There's probably no particular reason. Sets and props on long-running TV shows often change as needed and for various reasons throughout a series run.
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