Deliberate mistake: Whenever anyone walks across the front hallway, it takes them 6-7 seconds to walk from one stairway to the other. On the backside of the stairway, it only takes 1-2 seconds to walk across.
The White Asparagus Triangulation - S2-E9
Continuity mistake: When climbing the stairs at the start, Penny and Sheldon turn the corner and carry on the conversation. Camera cuts and now they turn the corner again and continue their conversation without breaking it.
The Griffin Equivalency - S2-E4
Continuity mistake: When talking to his parents, Raj's grip on the laptop changes between shots. Note the side of the screen. (00:18:10)
The Barbarian Sublimation - S2-E3
Continuity mistake: When Penny is talking to Sheldon in his bedroom, her hands are constantly changing positions between shots. (00:08:45)
Continuity mistake: Leonard says that Sheldon has gone crazy and chucks his pen down on a red pad on the desk. When the shot changes, so does the position of the pen without being touched. (00:13:10)
The Clean Room Infiltration - S8-E11
Other mistake: When Howard, Leonard, and Raj are arguing in the clean room, Howard says that Leonard is jealous because Howard and Leonard used to be "best friends until Koothrapali came along." Yet, in the earlier seasons, we know that Howard and Raj already knew each other when Leonard met them for the first time and brought them back to the apartment to play video games. Meaning they couldn't have been best friends before Raj came along.
The Werewolf Transformation - S5-E18
Character mistake: In S5-15, Sheldon is asking his friends to take him to SuperCuts. But in this episode, he goes to Mr. D'Onofrio's barber shop where he says he's gone to since he moved to CA, and he said D'Onofrio had his "haircut records."
The Hamburger Postulate - S1-E5
Continuity mistake: Leonard and Leslie are rehearsing the violin and cello in the guys' living room, and when they leave the room to go to Leonard's room, Leonard's cello and stand are still in front of the comfy chair. When Sheldon gets home, the cello and stand are gone, and only Leslie's closed violin case is left on the sofa. And she picks it up and leaves with it. How did the violin get back in the case.
The Retraction Reaction - S11-E2
Deliberate mistake: Leonard is on NPR on a Friday, and the guys don't talk about it until the following Monday at work. They spend so much time together, they surely would've discussed it sooner.
The Retraction Reaction - S11-E2
Character mistake: When Leonard arrives at the Human Resources office, he tells the HR person that he's not used to being called into the Dean's office. He's not in the Dean's office, he's in HR.
The Convergence Convergence - S9-E24
Other mistake: Howard believes he's being followed by the government, when it's actually Leonard, Penny, and Beverly in their car. The shots from Howard's car show only Leonard's silhouette. The other two do not appear as silhouettes.
The Higgs Boson Observation - S6-E3
Factual error: Sheldon tells Alex that he's run a background check, including her medical records. There is no way he could obtain medical records without a HIPAA authorization signed by Alex herself. It's illegal.
The Countdown Reflection - S5-E24
Continuity mistake: Penny mentions that she knows lots of people who got married in Las Vegas. In episode 7/9, she discovers that her "fake" wedding to Zach in Vegas was real.
Continuity mistake: Sheldon puts the bag of takeout into the lobby trash can and as he does so, the whole bag turns sideways. At the end of the scene, he takes the bag out of the can and it's upright without him turning it.
Continuity mistake: When Leonard is consoling Penny on the couch, the blue Afghan she has with her constantly changes position, including jumping from the floor back to the couch.
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.
The Colonization Application - S8-E17
Character mistake: Leonard and Penny were using the body paint the second time, the entire canvas is covered, and they each are covered with paint. But not a bit got into their hair.
The Hamburger Postulate - S1-E5
Continuity mistake: When Sheldon is explaining to Leonard why a Cheesecake Factory Hamburger is superior to a Big Boy Hamburger the number of fries on Leonard's plate noticeably changes between shots.
The Bow Tie Asymmetry - S11-E24
Continuity mistake: After the ceremony, as Amy and Sheldon are walking down the aisle, different camera angles show Barry Kripke holding the microphone in different hands but without changing them as he is singing.
The Apology Insufficiency - S4-E7
Continuity mistake: When Sheldon is drink his shots, the amount in his shot glass increases before he takes final shots.
Answer: Because Raj's selective mutism seems to be caused by women that Raj sees (on some sort of conscious or subconscious level) as a potential sexual partner. An attribute that wouldn't apply to either his sister or mother.