The Killer Robot Instability - S2-E12
Factual error: M.O.N.T.E. would not be able to knock down the door as he does at the end.
The Killer Robot Instability - S2-E12
Revealing mistake: Near the end of the show, the big robot is chasing the other robot down the hallway. The camera starts to zoom in as the robots come by it. The big robot stops just before the camera zoom is past it, yet they keep running down the hallway like the robot is still going.
The Killer Robot Instability - S2-E12
Revealing mistake: When M.O.N.T.E. crashes through the door at the end, the door hits the wall on the other side of the corridor. The wall shakes, revealing it's a set.
The Killer Robot Instability - S2-E12
Revealing mistake: When M.O.N.T.E. crashes through the door, if you watch carefully, the door is already off its hinges and falling before M.O.N.T.E. hits it.
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)
The Maternal Capacitance - S2-E15
Continuity mistake: After Leonard's mother says, 'Tell me about it', her head was tilted downwards, but in the next shot, she was looking straight again. (00:04:35)
The Maternal Capacitance - S2-E15
Character mistake: Leonard has his phone on his left ear. Sheldon says "Since ears don't cross hemispheres..." This is an error. The vast majority of the input to the left (or right) ear crosses over to the opposite hemisphere.
The Cushion Saturation - S2-E16
Factual error: Leslie Winkle takes shelter with Howard in the middle of a paintball game. Her gun has no hopper for paintballs and thus no ammo, so it's useless, despite her apparently using it outside seconds earlier. This isn't a model with an internal magazine - the port to attach the missing hopper is plainly visible.
The Cushion Saturation - S2-E16
Continuity mistake: When Howard and Leslie are in the paintball shed, Howard's paintball ammo pack is closed, but when he and Leslie start kissing, it's open.
The Work Song Nanocluster - S2-E18
Continuity mistake: When Penny puts the box down at the start it is closed. Camera cuts and it's open instantly.
The Work Song Nanocluster - S2-E18
Continuity mistake: When Penny gets the package, the position of the box on the counter keeps changing position.
The Dead Hooker Juxtaposition - S2-E19
Continuity mistake: In the scene after the boys have followed the new neighbor out and Sheldon and Penny are left eating alone, the utensils Penny holds change from chopsticks to a plastic fork and back several times.
The Dead Hooker Juxtaposition - S2-E19
Continuity mistake: After the guys leave and Penny and Sheldon are eating alone, Sheldon's chopsticks switch between an unevenly broken pair of chopsticks and an evenly broken pair of chopsticks.
The Hofstadter Isotope - S2-E20
Continuity mistake: Near the end of a conversation with Stewart, Penny is holding a drawing that Stewart drew for her. In the next shot, it is now on the counter and she is picking it up.
The Vegas Renormalization - S2-E21
Continuity mistake: When Leonard and Raj are getting ready in the bathroom, Raj picks up a tube of shampoo/conditioner off the bathroom bench with his left hand. However, in the next shot, Raj has nothing in his left hand and the tube is back on the bench, with no time for him to replace it between shots.
The Vegas Renormalization - S2-E21
Continuity mistake: In the bathroom of the hotel in Las Vegas, we can see Rajesh taking a small white bottle and keeping it in his hand while saying "I love Las Vegas". In the next shot, the bottle has returned to its place instantly.
The Classified Materials Turbulence - S2-E22
Continuity mistake: In the comic book store, when Stuart and Leonard are talking about Penny saying "Leonard" when with Stuart, Stuart's pen/hand is constantly changing position instantly.
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.