Gives a rare camera view of Leonard and Sheldon's apartment, from the vantage of the hallway that's normally seen to the back right, going to the bedrooms, allowing us to see the "4th wall" in the front of the apartment the camera normally sees through.
Sheldon is correct in his assertion that Second Street is the most common street name in America. This is according to The National League of Cities which lists 10,866 streets with that name. First Street is actually third on the list behind Third Street.
The shots taken from Sheldon's viewpoint as he enters the living room was recorded the day previous to "show day." This is because putting up and taking down the "fourth wall" would have been too time consuming to accomplish in front of a studio audiences. Shooting scenes ahead of time is common practice in shows "shot before a studio audience," either due to time constraints, inaccessibility of the shooting location to the audience, or other mitigating factors (such as nudity).
The first proper appearance of Sheldon's signature triple knock: "Knock-knock-knock, Penny? Knock-knock-knock, Penny? Knock-knock-knock, Penny?" He used a similar knock in The Loobenfeld Decay (2008), but it consisted of four knocks followed by a name, rather than three knocks.