The Big Bang Theory: 15 Insane Details You Definitely Missed
12. The Apartments Have Doorbells That Are Never Used
A great detail hidden in plain sight of the viewer, the apartments rented by Penny, and Leonard and Sheldon - and subsequently by Penny and Leonard, and Sheldon and Amy - each have a doorbell that is never used.
Their existence is entirely ignored; much like the elevator, the lack of the doorbells' use could be explained to the viewer amongst a regular discussion or, if needs must, in a flashback scene.
Perhaps they were rendered useless following the elevator's explosion? Either way, their nonexistence means Sheldon gets to carry on his three-knock procedure.