The Griffin Equivalency - S2-E4
Continuity mistake: When discussing what qualities their new friend should have, the container Howard has changes position instantly. (00:17:20)
Ssiscool
The Griffin Equivalency - S2-E4
Continuity mistake: In the opening scene, just after Howard says, 'Creepy good or creepy bad?', there is a shot where Howard is holding his food, even though Leonard hasn't given it to him yet and it isn't there in the shot before or after. After Leonard gives the food to Howard, the exact same shot is used after he says, 'I'm not necessarily talking about the food', so the shot was taken from there and it is shown twice. (00:00:25)
duncanmc
The Griffin Equivalency - S2-E4
Continuity mistake: When Raj asks the guys who's going to the party on Saturday night, Penny's grip on the martini glass she has changes between shots. (00:10:20)
Ssiscool 
The Griffin Equivalency - S2-E4
Continuity mistake: When talking about wanting a griffin, the bottles and strip of paper on the table keep moving between shots. (00:16:00)
Ssiscool 
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.