The Contractual Obligation Implementation - S6-E18
Factual error: At the end of the episode, the girls are at Disneyland in costume as Disney Princesses. Adults at Disneyland are not permitted to wear character costumes because of the risk of them masquerading as Cast Members, so they certainly would not have been dressed that way at Bibbidy Bobbidy Boutique.
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 Electric Can Opener Fluctuation - S3-E1
Continuity mistake: Sheldon's Sandwich goes from looking burnt to not burnt in the next few frames. (00:16:56)
The Transporter Malfunction - S5-E20
Continuity mistake: Sheldon opens his transporter box and takes out Mr. Spock, while making a big deal about it remaining mint in box, unopened. Later, he makes a big show of using an exacto knife to gently open the box with Spock in it.
The Vengeance Formulation - S3-E9
Character mistake: When Leonard, Sheldon, and Raj are watching Kripke entering his office with the board of directors, Kripke says "President Siebert", yet Siebert is not among the people standing with him.
Continuity mistake: When Sheldon is looking at his socks that Penny paired up, his grip on them changes between shots. (00:06:00)
Continuity mistake: Penny throws Sheldon's sock down on the table, and their positions change when Sheldon picks them up, despite no one touching them. (00:05:50)
The Cooper-Nowitzki Theorem - S2-E6
Continuity mistake: When Raj says there is always a catch, Leonard's hand goes from on the table to on his leg between camera cuts. (00:03:00)
Continuity mistake: When Leonard shows Penny the leaflet at the end, she is holding it in one hand in shot but with 2 as the camera changes. (00:19:50)
Continuity mistake: In the laundry room, Sheldon is holding a blue shirt in his hands and has a bit of extra material near his hands. When the shot changes, he is holding the shirt differently and the extra material has gone. (00:05:50)
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.