The Bat Jar Conjecture - S1-E13
Continuity mistake: At the end, when Leonard is gloating over beating Sheldon, his position on the sofa keeps changing in relation to the arm. (00:18:50)
The Bat Jar Conjecture - S1-E13
Continuity mistake: When Raj is talking about psychological warfare he has a fork in his hand in one shot and not in the next. (00:08:40)
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.