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.
Suggested correction: Presumably Sheldon also buys non-allergenic cats.
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: Throughout the early episodes, shots of Sheldon's bathroom shows the door open and a rack of towels against the wall. Later episodes show that that area is actually where the sink and mirror are.
Continuity mistake: In "The Luminous Fish Effect" (S1E04) Sheldon tells Penny he weighs 140 pounds. In "The Porkchop Indeterminacy" (S1E16), he tells his sister he weighs 165. 25lbs difference would be a noticeable change, but he looks the same.
Suggested correction: Lying to Penny but not to his sister, not inconsistent with the characters and his relationships.
The Big Bran Hypothesis - S1-E2
Continuity mistake: When Leonard and Sheldon are pushing the box up the stairs, the position of the box on the stairs changes instantly. In one shot it is flush against the wall, then in the centre of the stairs. (00:05:20)
