In Sheldon and Amy's apartment, there's a Harley Quinn comic book on the coffee table. Kaley Cuoco (Penny) would go on to voice Harley Quinn in the HBO Max series Harley Quinn (2019).
Ironically, Howard doesn't recognize Neil Gaiman, even though he is one of his favorite authors and one of the 'Neils' that he named his son after in The Neonatal Nomenclature (2018).
When Amy introduces herself as Sheldon's fiancee, Denise says that she was unsure if Amy was real. In The Sales Call Sublimation (2016), Leonard mentioned Sheldon to a psychiatrist, who later admits to Penny that she wasn't quite sure if this "Sheldon" was real.
Raj's telescope is a popular model of Schmidt-Cassegrain design. The brand name, "CELESTRON," is partially concealed so that it appears to be "LESTRO."