The Russian Rocket Reaction - S5-E5
Character mistake: Sheldon says he drove for hours in a bus to meet Wil Wheaton. He meant he rode on a bus. Sheldon never would've made that semantic error.
The Russian Rocket Reaction - S5-E5
Continuity mistake: Bernadette holds her wine glass in her left hand, holding it from the base of the glass; in the next shot she's drinking from her right hand holding the top of the glass and in the immediate next shot she's once more holding the glass from her left hand. (00:14:40)
The Russian Rocket Reaction - S5-E5
Continuity mistake: In the scene where Howard and Bernadette are about to have sex and Bernadette leaves to "brush her teeth", Howard stretches and lays on the bed uncovered. After Howard's mother yells "over my dead body my son goes to space" and Bernadette appears again and says "I'm ready", then it flashes back to Howard and he is instantly covered up.
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.