The White Asparagus Triangulation - S2-E9
Continuity mistake: When Sheldon is in Penny's apartment trying to get her to tell him why her relationship with Leonard failed, the yellow tee-shirt on the red pillow next to him keeps changing position.
The White Asparagus Triangulation - S2-E9
Continuity mistake: When Leonard is helping Sheldon open the jar, the red napkin which he left on the couch where he was sitting changes position or vanishes in every shot.
The White Asparagus Triangulation - S2-E9
Continuity mistake: When Sheldon is questioning Raj and Howard in the cafeteria, the bottle of water is on Raj's tray faces the camera with the barcoded side. But in one closeup it rotates to show the logo-branded side.
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.