At the prom on the roof, Penny is wearing the same prom dress she wore in high school, the one that Amy picked up from Penny's couch at the beginning of the episode.
Sheldon said while trying to figure out his feelings of love for Amy, he said he considered he had an alien parasite. In season 4 episode 10, Amy got aroused by Penny's old boyfriend Zack. While trying to figure out the feelings, Sheldon and Amy considered she had an alien parasite.
This episode was taped on October 14, 2014.
The black lace pattern on Jeanie's yellow dress is similar to the black lace pattern on Amy's yellow & black dress in The Focus Attenuation (2014).