The Bath Item Gift Hypothesis - S2-E11
Continuity mistake: When Penny takes a drink from the egg-nog carton before kissing Leonard on the cheek, between shots the logo-branded label changes which direction it faces in her hand. (00:16:45)
The Bath Item Gift Hypothesis - S2-E11
Continuity mistake: When Leonard gives Penny her christmas gift, as she unwraps it she tears the paper off so that most of the box is exposed. In the next shot much more of the wrapping is back intact and differently positioned. (00:19:40)
The Bath Item Gift Hypothesis - S2-E11
Continuity mistake: When Sheldon emerges with all of the bath item gift baskets for Penny, he drops them on the floor and chair. Between shots one of the baskets on the chair changes from being an upright green-ribboned basket, to a toppled-over white-ribboned basket. (00:19:50)
The Bath Item Gift Hypothesis - S2-E11
Continuity mistake: Near the start, when the camera is on Raj, David Underhill is seen in the top left-hand corner of the screen shaking someone's hand, already in the cafeteria. Then, when the camera is on Wolowitz, David is just walking into the door of the cafeteria. David sits down to the right of the man sitting at the table, then shortly afterwards David is now sitting where the other guy was, originally.
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.