The Lizard-Spock Expansion - S2-E8
Continuity mistake: When the guys are in the Mars Rover test room, the time displayed on the large digital clock seen behind Howard jumps about inconsistently between each shot. (00:07:50)
The Lizard-Spock Expansion - S2-E8
Continuity mistake: At the very beginning, Leonard's Diet Coke keeps changing position without anyone touching it.
The Lizard-Spock Expansion - S2-E8
Continuity mistake: When Leonard is talking to Penny in the laundry room; as he walks forward to ask her about whether him dating Stephanie is wrong, the box of washing powder on the machine behind him alternately has its lid closed or open. (00:14:00)
The Lizard-Spock Expansion - S2-E8
Continuity mistake: When Raj says Star Trek 5 is worse than Star Trek 1, his hands are outspread. Camera cuts and now they are clasped together. (00:04:15)
The Lizard-Spock Expansion - S2-E8
Continuity mistake: When Sheldon goes to speak to Leonard, he is sitting on the couch. Camera cuts to Raj for one second, then to Penny and you can see Sheldon has crossed the apartment and is now in the hall. There is not enough time to move that far in that time. (00:09:05)
The Lizard-Spock Expansion - S2-E8
Continuity mistake: The way Leonard holds the can of Diet coke before he puts it down changes between shots. (00:00:25)
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.