The Big Bran Hypothesis - S1-E2
Continuity mistake: At the end of the episode, when the gang is reading the furniture assembly instructions, a pencil in the cup in front of them suddenly vanishes. (00:19:00)
The Creepy Candy Coating Corollary - S3-E5
Continuity mistake: At the dinner, Howard's mom calls. Howard's wine glass refills itself between shots. (00:15:20)
The Luminous Fish Effect - S1-E4
Continuity mistake: When Penny comes into the apartment to ask if Sheldon needs anything from the market, between the few shots, the position of the napkin and the pile of eggs on the plate he is holding changes. (00:05:15)
The Large Hadron Collision - S3-E15
Continuity mistake: Penny asks Sheldon if the roommate agreement contains something about girlfriends, which Sheldon dismisses as unrealistic. However, it was revealed in season 2 that there is a cohabitation rider. This was revealed when Sheldon asserted that Leonard's girlfriend, Stephanie, was officially living with them.
The Robotic Manipulation - S4-E1
Continuity mistake: In the ER, the glasses around the nurse's neck move instantly without her touching them. They go from angled to straight instantly. (00:17:45)
The Fuzzy Boots Corollary - S1-E3
Continuity mistake: When Sheldon, Howard and Raj play Jenga, there is one block sitting at the top of the tower. When Leonard walks in, there are now two blocks at the top, before we actually see Howard place the second one on the top. (00:08:10)
Continuity mistake: When Penny is squirting chocolate syrup in the ice cream container, she drizzles some on the lip of the carton, causing it to run down the outside. The camera switches to Leonard and then back to Penny, at which point, the chocolate is gone. (00:04:50)
The Wildebeest Implementation - S4-E22
Continuity mistake: When Howard, Bernadette, Leonard and Priya are playing Jenga on the coffee table, the silver coffee pot, between the game and a coffee cup, disappears and reappears between shots.
The Irish Pub Formulation - S4-E6
Continuity mistake: As Raj, Howard and Sheldon are sitting at the table discussing leaving Priya alone while Leonard and Priya are away from the table, a water bottle sitting in front of Raj moves back and forth between his tray and Leonard's tray.
The Luminous Fish Effect - S1-E4
Continuity mistake: When Leonard and Sheldon are walking downstairs, Sheldon stops and pins back up the caution tape that is in front of the elevator. Between shots, the other strip of caution tape untwists itself. (00:01:25)
The Higgs Boson Observation - S6-E3
Continuity mistake: At the end, when Alex and Sheldon are at the apartment, when Alex says "she seems nice", there are two beer bottles on the coffee table. The camera goes to Sheldon, and when it returns to Alex, there is only one bottle.
The Dead Hooker Juxtaposition - S2-E19
Continuity mistake: In the scene after the boys have followed the new neighbor out and Sheldon and Penny are left eating alone, the utensils Penny holds change from chopsticks to a plastic fork and back several times.
The Cohabitation Formulation - S4-E16
Continuity mistake: When Penny is pouring water into the glasses on the table after first meeting Raj's sister. The level of water in the pitcher changes between shots.
The Luminous Fish Effect - S1-E4
Continuity mistake: When Sheldon's mom is saying the prayer, the bread roll that Raj is holding swaps hands, and then it disappears in the shot where she tells them they don't have to join in with the end of the prayer.
The Cooper Extraction - S7-E11
Continuity mistake: When Amy says that Penny never would have met Stan Lee, she draws a thick black line between "Penny" and "Stan Lee" on the board. The camera goes to Penny, and when it comes back to Amy, the line is blue and faint.
The Financial Permeability - S2-E14
Continuity mistake: When the guys are using the whiteboard to figure out their cinema plans, Sheldon draws two black X's on the board with a marker. In the following shots the appearance of the X's changes. (00:00:30)
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.