The Flash t-shirt Sheldon is wearing after he decided to get back to his lifestyle in 2003 before meeting Leonard is the same t-shirt that he wore when he met Leonard for the first time in the origins episode The Staircase Implementation (2010).
Penny has a hard time containing her laughter with Raj and Howard's song. This was not scripted but Kaley Cuoco breaking character, as can be seen in the blooper reel for season 9, where everyone bursts out in laughter at one point.
Laura Spencer is now promoted to the main cast as her name appears in the opening credits, but is only credited in the episodes she appears in, just like Kevin Sussman as Stuart.
Title Reference: The title refers to Sheldon wanting to return to the unemotional state of his personality before he met Leonard in 2003.