The Higgs Boson Observation - S6-E3
Other mistake: Just before Amy video chats Sheldon there is a shot where she is wearing glasses, and when the camera zooms out she isn't.
The Parking Spot Escalation - S6-E9
Other mistake: Sheldon tries and fails to move Howard's car at the start of the episode. However if you look closely the car does roll backwards even though the handbrake is applied.
Other mistake: When Howard, Stuart and Sheldon are ringing the bells, they are ringing differently from the "code" (00:16:45)
The Spoiler Alert Segmentation - S6-E15
Other mistake: When Leonard and Sheldon are about to make up, you see green lights flash in the hallway. The next shot you see the hallway and it's completely normal.
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.