People often visit the two apartments, knocking on the door or walking in (Howard and Raj, Amy before she moved in, food delivery, other visitors). However, the building has a locked door, such as is demonstrated in Season 11 Episode 18, The Gates Excitation (2018), where Sheldon doesn't hold the door for Leonard who has to then use his key to get into the building. How come ALL of these people are able to just get into the building without being let in? They never get buzzed in, visitors often surprise them (such as Sheldon's mystery date in The Mystery Date Observation (2015)).
Penny's first car is a red 1980's Volkswagen Cabriolet Mk1 convertible. In numerous episodes it is mentioned to have the car's "check engine" light on. 1980's VWs (without computerized engine controls) had no "check engine" lights.
A line of the opening song states that neanderthals developed tools. Homo habilis actually developed tools long before the neanderthals.
There are several chronological errors in the pictorial time-line shown during the musical title sequence. For example, the Moai of Easter Island are shown as "BC," and the Giza Pyramids follow the Chichen Itza El Castillo.
There are 16 mailboxes but the building has only eight apartments - two on each of the five floors, with the first floor having none. Since the missing fourth wall has been established as being THE outer wall - Sheldon's favorite spot is partially because he can open a window in this wall to create a cross-breeze - there are no other apartments, only 2 per floor, it is not possible that there is a never seen third or fourth apartment on each floor. There are no apartments on the first floor, under Penny's is the building's front door, and under the guys' are the mailboxes. It has been shown that there is only one more floor before the roof, above the one Sheldon, Leonard, Penny, and later Amy live on.
Sheldon often says that he has an eidetic memory like that of Wesley Crusher and can remember everything that has ever happened to him. However, it is never mentioned on Star Trek: The Next Generation (1987) that Wesley has such a memory, while it is mentioned frequently that he is an exceptionally gifted genius.
In one episode, Sheldon states that pets are banned under the roommate agreement, but in another he adopts 25 cats. But it's regularly shown that Sheldon uses the roommate agreement to his own advantage, sometimes unaware, but more often completely ignoring, how it affects Leonard.
Pasadena is in Los Angeles county which has extremely high rent prices. The median rent for a one-bedroom apartment was about eleven hundred dollars in the 40th-precentile in 2008 and a lot more for a nice one like Penny's. She was never going to swing that by just being a waitress. She managed to by cutting costs, like sponging off Sheldon and Leonard's Wi-Fi, buying car insurance from a third world country, paying a bill with what she had and a picture of her in her underwear, etc. People come to Los Angeles to become actors all the time, and they find a way, and most people who do come to Southern California are very much aware of the high cost there, and if they don't, they find out really quickly.
The theme song states "The autotrophs began to drool"; autotrophs are plants and do not drool. However, the line is meant to be taken as a metaphor for evolution which posits that the earliest organisms must have been autotrophs and that the heterotrophs evolved from them at a later stage.
Whenever anyone is using the stairs in the apartment building, it is obvious that each floor is the same set, slightly redecorated. This can be most easily seen in the distinctive marks on the lamps that are next to each apartment doors (especially the one on the left.)
The front doors of the apartment building are even with Penny's apartment door, meaning her apartment - and those above and below - hang out in space over the front sidewalk.
Sometimes there's a fire alarm and emergency instructions notice next to the elevator and sometimes there isn't.
Every time that Sheldon plays the recorder, he plays it incorrectly.
Throughout the run of the series everyone knocks on the doors of the apartments in Leonard and Sheldon's building, yet each has a doorbell button next to the door.
Regarding the swinging door between the living room and the kitchen in the Wolowitz house, the door is hinged on the right side from the perspective of both the living room and the kitchen.
In the time-line at the beginning of the show there is a that says 91,0009C instead of 91,000BC.
In one of the episodes when Sheldon moves in with Raj, Raj is watching an Indian movie. The actress of that movie is referred to as Aishwarya Rai Bachchan, but it's actually Amisha Patel.
Sheldon's trademark knock (3 knocks and name, 3 knocks and name, 3 knocks and name) was not established until later in the first season; however, in a scene that pre dates the start of the show, he is shown doing that knock sequence.
In one of the episodes Sheldon mentions the male Indian peafowl as the male peacock. That is a common mistake people make. The correct name for the bird is Indian peafowl. The male is a peacock and the female is a peahen.
Since Sheldon is an exaggerated and ongoing germaphobe, he would never use a public washing machine nor dryer.