I think this will explain it. I found it on IMDB"Dick Van Dyke was granted a one-week leave from "Bye Bye Birdie" to film the show's pilot episode. During his absence, understudy Charles Nelson Reilly filled in. When Van Dyke left "Bye Bye Birdie" for good in April of 1961, he was succeeded by the future host of "The Match Game" (1962), Gene Rayburn. Van Dyke was so nervous about filming the pilot that he developed a cold sore on his upper lip, requiring additional makeup to cover it up, and he admits he cannot recall President John F. Kennedy being inaugurated on the same day the pilot was filmed."
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