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.
Factual error: Characters sometimes talk to one another on video calls, and a character (A) often watches these from an angle rather than face on, allowing the viewer to see both character A and the character on the call (B). When this happens, character B is shown turned a little to one side, to give the impression that they are looking directly at character A (as they would be if they were present in the same room). In reality, this means that character B is looking off to one side of their monitor and not at the image of character A, something that it makes no sense to do. No matter what angle the laptop is shown from, provided character B is visible, they should appear to be facing the viewer.
Suggested correction: Since we almost always see these video calls using full-screen, people sitting side-by-side would be on different sides of the screen. You wouldn't look into the webcam itself to see them. You would look at the screen, and move your head slightly to look at the left part versus the right.
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.
The Einstein Approximation - S3-E14
Character mistake: Sheldon is an extreme germophobe to the extent he doesn't even shake hands. There is no way he would be climbing around in a ball pit.
Suggested correction: If he was acting normally (for Sheldon), he wouldn't. However, Sheldon isn't acting as he normally would due to lack of sleep, and his obsession with solving a problem.
The severity of his phobia would not be minimized by lack of sleep.
Sheldon's mind is distracted by the problem he is trying to solve. He grabs other people's food, he allows Raj to touch his food, he handles dirty cutlery at the cheesecake factory, All characteristics that would be present if he was acting normally. The fact that he is in the ball pit is simply an extension.
Severe sleep deprivation could certainly affect how Sheldon reacts to phobias. The brain requires sleep to function and with a severe lack of sleep, the brain is no longer able to send the signals that alert Sheldon to his fear of germs.
Continuity mistake: Raj talks in front of Penny near the beginning of the episode, then picks up dishes, walks toward the kitchen, and just disappears from the set.
Suggested correction: Couple of points here, 1. Raj talking in front of Penny is already listed. 2. Raj doesn't pick the plates up until Penny has left after the Halo game. 3. He then stays in the kitchen area while we see Penny come back and settle down to go to sleep and Leonard and Sheldon go to bed and turn the lights out. Raj then creeps out the apartment and scares Penny.
The Creepy Candy Coating Corollary - S3-E5
Continuity mistake: Bernadette speaks regular here. In future episodes she suddenly has a higher-pitched voice.
Suggested correction: This isn't really so much a mistake as it was a character tweaking, which happens often in TV series.
The Big Bran Hypothesis - S1-E2
Other mistake: Leonard is awoken by the apartment door closing. When he walks into the living room the door is open. (00:08:55)
Suggested correction: He's awoken by a door closing, not necessarily the front door. He could have heard Sheldon closing his door.
The Launch Acceleration - S5-E23
Factual error: In all the episodes leading up to Howard and Bernadette's wedding, Howard was going into space to install a space toilet. But in this episode, Howard tells her they want his telescope, so he has to go.
Suggested correction: The space toilet was something Howard designed that NASA installed back in season 2. His mission to the ISS in season 5 was always about his telescope. In episode 5, "The Russian Rocket Reaction", Howard says NASA picked his team's design for the deep field space telescope that's going on the International Space Station in the spring.
The Big Bran Hypothesis - S1-E2
Character mistake: Sheldon does not do his 3x OCD knock on Penny's door.
Suggested correction: That quirk wasn't brought into the series until halfway through the first season. The one-knock Sheldon happened a couple of times before it became the three knock habit.
