The Financial Permeability - S2-E14
Continuity mistake: When the guys are using the whiteboard to figure out their cinema plans, Sheldon draws two black X's on the board with a marker. In the following shots the appearance of the X's changes. (00:00:30)
The Financial Permeability - S2-E14
Continuity mistake: Sheldon opens the can with money in and puts the lid on the side, However in some shots he has the lid in his hand. (00:04:00)
The Financial Permeability - S2-E14
Continuity mistake: When Sheldon is holding the money out to Penny, the can that is being held by Sheldon is constantly moving up and down in height in relation to his shirt. (00:04:00)
The Financial Permeability - S2-E14
Continuity mistake: When Sheldon asks Penny if he lent her enough money, the container he is holding opens on its own. (00:09:25)
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.