The Barbarian Sublimation - S2-E3
Revealing mistake: When Sheldon gets the sword for Penny in the game, Penny walks out and you can see the laptop isn't switched on. (00:07:55)
The Euclid Alternative - S2-E5
Revealing mistake: When we see Sheldon and Howard on the scooter, look in the back of the mirrors to see the reflection isn't moving. Indicating the bike isn't moving. (00:07:40)
The Killer Robot Instability - S2-E12
Revealing mistake: Near the end of the show, the big robot is chasing the other robot down the hallway. The camera starts to zoom in as the robots come by it. The big robot stops just before the camera zoom is past it, yet they keep running down the hallway like the robot is still going.
The Killer Robot Instability - S2-E12
Revealing mistake: When M.O.N.T.E. crashes through the door at the end, the door hits the wall on the other side of the corridor. The wall shakes, revealing it's a set.
The Killer Robot Instability - S2-E12
Revealing mistake: When M.O.N.T.E. crashes through the door, if you watch carefully, the door is already off its hinges and falling before M.O.N.T.E. hits it.
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.