Throughout some episodes of The Big Bang Theory, there have been some contradictions, which include the following:

• Sheldon is sarcastic to Leonard, telling him that "Archimedes would be so proud" in the Pilot episode when Leonard fails after invoking the mathematician's name and teachings. In the next episode, however, it is made apparent several times that Sheldon does not understand sarcasm and even needs a SARCASM sign. It may possibly be that Sheldon knows what sarcasm is, but he can't tell when other people are using it.

• 

  In The Fuzzy Boots Corollary, Leonard mentions Sheldon gets asthma when he is around cats, but Sheldon said his family had a pet cat named Lucky and bought 25 cats when Amy "broke up" with him.
• In The Maternal Congruence, it is said that Sheldon cheers for the Grinch's meanness in its movie and finds it a buzz kill when he succumbs to social convention and gives the presents back. In a different episode, Sheldon compares Stuart to "the Grinch in Christmas" in a bad way, showing love for Christmas and hate for the Grinch.
• Early in Season 1, Sheldon tells Penny that he is 140 pounds when he goes to the market with her. However, near the end of season 1, he tells his sister to tell their mother that he is 165 pounds, among other things, which would be a 25 pound increase in only a few months.
• In The Luminous Fish Effect, Sheldon estimates that the car he and Penny are riding in is about 4000 pounds. This cannot be correct, as it is a Volkswagen Rabbit (Golf Mk 1) and would be approximately 2000 pounds, maximum.
• Howard is described to be 97 pounds and 108 pounds at different points.
• Sheldon recalls how he made Leonard wait in line for Star Trek Nemesis to get good seats, though that film was released in 2002 and Leonard and Sheldon didn't meet until 2003, according to the flashback episodes. Given the tepid response to the film by the public at large, it's unlikely there was a revival showing of it, unless it too had unseen footage, that would have made the movie better.
• Sheldon is always telling his friends that he does not like touching other people - however, he has made physical contact with Penny (hugging her) and Amy (cuddling), without washing his hands afterwards, or being utterly revolted.
• In The Luminous Fish Effect, Sheldon gave a complicated explanation of what he did with his life past fifth grade, which combined would make him 23 at that time. However, in The Friendship Contraction , Leonard stated that Sheldon was 30. Given the information in The Luminous Fish Effect, Sheldon would only be 27.

  

• In The Porkchop Indeterminacy, Sheldon's sister, Missy is seen as someone who speaks with a heavy Texan accent. However, there are several instances in the episode where she is seen to have dropped her accent.
• In The Love Car Displacement, Amy's name tent did not have the title "Dr" in front of her name, while Sheldon's, Leonard's, Raj's did and obviously Howard didn't. This is clearly a contradiction since Amy had been revealed to have a PhD before this episode.
• In The Good Guy Fluctuation Stuart congradulates Leonard for being the first person to pick up a girl in Stuart's comic book store. However, Stuart himself picked up a girl (Penny) in that same comic book store in an Season 2 episode, The Hofstadter Isotope.