Audio problem: When Sheldon is knocking on Leonard's door at the start, the sound is out of sync with the action.
The Hamburger Postulate - S1-E5
Audio problem: When Leonard and Leslie continue to practise together after the other two musicians have left (Leslie now being the only violinist), vibrato from a violin can be heard - from observing Leslie's left hand she is not applying it.
The Big Bran Hypothesis - S1-E2
Audio problem: When Penny is talking to Raj on the stairs to the apartments, during some shots we see her from behind. In one shot, we hear her say something like "And I'm freaking out", but you can tell by the way her jaw moves that she was actually saying something different. (00:14:50)
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.