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 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 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.