The Beta Test Initiation - S5-E14
Factual error: Penny picks up a hand gun, pulls back the slide to ensure the chamber is empty (which would discharge any round in the chamber too) and then releases it, leaving it closed. She then loads a magazine into the butt of the weapon and hands it to Leonard. He starts to smooch her and shoots himself in the foot. The gun could not have fired, because Penny would have to pull the slide all the way back and then release it again to load the first bullet. There couldn't have already been a bullet in the chamber, as the whole reason Penny pulled the slide back in the first place was to check just that.
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.