The new Avengers movie the gang is going to see is Avengers: Endgame (2019), which had been released three days before this episode's broadcast. Bernadette says that she wants to see the movie in 3D because she wants to feel close enough to lick Thor's abs; she will be disappointed, because in the movie, Thor sported a large beer-belly.
This episode shows a close up of the bottom step in the hall (when Leonard is sitting on it), with a white paint mark on the wood part of the step, to the right-side of the carpet; this is also visible on the bottom step of the other floors, thereby confirming that they redressed the same set to show the different floors.
Although Leonard has sat in Sheldon's spot before (Sheldon even scared him off it in The Good Guy Fluctuation (2011)), this is the first time that he willfully sits there to defy Sheldon. When Sheldon storms out, it is the second time that Leonard declares that pissing Sheldon off is more satisfying than sex, after Priya found loopholes in Sheldon's Roommate Agreements in The Agreement Dissection (2011).
This is a near polar opposite of The Occupation Recalibration (2014), where Penny quits the Cheesecake Factory to pursue her acting career. Instead of Leonard freaking out about Penny quitting her job, Penny freaks out about Leonard quitting his.
Penny mentions that she can barely get Leonard to go to a beach. In The Zarnecki Incursion (2011), Penny stated that she spent two years trying to get Leonard to even go to a beach, and she had to carry him to the water because he was afraid to step on medical waste. And in The Anxiety Optimization (2015), she said that Leonard uses the time on a beach to walk around with his metal detector.