Continuity mistake: In the beginning of the series Sheldon knocks on Penny's door only once. Later in the series Sheldon explains he has always knocked 3 times because he walked in on his father with another woman.
Continuity mistake: In the early episodes, Leonard is depressed and buys cat stuff. He comments that he has taken in to account Sheldon's asthma with a hyper allergenic cat. Later in the series it is Leonard with asthma and Sheldon who gets all the cats.
Upvote valid corrections to help move entries into the corrections section.
Suggested correction: Presumably Sheldon also buys non-allergenic cats.
Suggested correction: Sheldon goes through phases of his mannerisms. This is pointed out a few times across the show.
Ssiscool