This is the second time that Penny does Sheldon's knock to him on his apartment door within six years. The first time was in The Panty Piñata Polarization (2008) (S2E7).
Raj blames Howard for taking Sheldon to Texas and showing him around NASA, which occurred in The Friendship Turbulence (2014); Sheldon and Leonard going to Skywalker Ranch without Raj and Howard happened in The Skywalker Incursion (2015). The other guys also blame Sheldon for spending a day with James Earl Jones without telling them (The Convention Conundrum (2014)); in all fairness, Sheldon told them he was going to meet up with Jones but the others simply thought that Sheldon would end up annoying him.
This episode shows one of only a few times that Penny is going to be expected to pay for dinner. At the end of the episode, she states that she doesn't know if she ever thanked Bernadette properly for getting her the job as a pharmaceutical sales rep. Bernadette suggests a thank you dinner and even reminds Penny to not forget her wallet as they are leaving. It's a long-running plot point that Penny doesn't pay for food as she is often seen eating in Sheldon and Leonard's apartment for free.
Title Reference: The title concerns the communication system that Leonard and Raj are developing for NASA to contact aliens and the breakdown of communication between them and the other guys.