Amy is still away at a convention and only shows up in a few brief scenes on a monitor in this episode and the previous one, so that Mayim Bialik could take some time off from the show.
Although the ending of The Hook-up Reverberation (2014) seemed to suggest that Penny and Emily still didn't get along, they appear to be on good terms now, as they are including Emily in girls' nights now.
When Leonard says that Sheldon needs to apologize to everyone he made feel bad when he was sick Sheldon says "Challenge accepted." That happens to be a catch phrase of Barney Stinson from How I Met Your Mother which was a role Jim Parsons who plays Sheldon auditioned for.
This episode was taped on December 15, 2015.