Sheldon (Jim Parsons) didn't always have his door-knocking quirk. In season one, episode five, The Big Bang Theory: The Hamburger Postulate (2007), he knocks on Penny (Kaley Cuoco)'s door without knocking and calling her three times.
Sheldon's trademark knock (3 knocks + name, 3 knocks + name, 3 knocks + name) was not established until later in the first season, however, in a scene that pre-dates the start of the show, he is shown doing that knock sequence.
One of the final cards to appear in the credit sequence is the vanity card for Chuck Lorre Productions. In keeping with his practice on Dharma & Greg (1997) and Two and a Half Men (2003), there is a different message on the card each week, written by Chuck Lorre himself.
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