
Continuity mistake: In the electronics store, when the man asks Sheldon which hard drive he should buy in the first shot he is holding the green box in his left hand. Next shot it is under his arm. (00:12:50)

Continuity mistake: When Penny asks Sheldon "What trauma?" in the electronics store, Sheldon holds two boxes - one with an orange side on top of one with a black side. In the next shot the black sided box is on top of the orange one. The boxes then continue to swap places during the whole scene. (00:11:50)
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.