Leslie Winkle returns and makes a special guest appearance. She was never mentioned between her previous appearance, season 3's The Lunar Excitation (2010), and this episode.
Since this is the 200th episode in the series, rumors circulated for weeks as to which celebrities would appear as guest stars on the episode.
With Leonard, Penny and Leslie Winkle present in one room, it reunites Leonard with two of his love interests on the show (one current, one former), as well as Johnny Galecki with two of his real-life ex-girlfriends (Kaley Cuoco and Sara Gilbert).
Strangely, Sheldon's family is absent from his birthday party, including his mother, sister, brother and Meemaw. The reason is probably that they are celebrating Missy's birthday party (since she is Sheldon's twin sister).
Sheldon again shows his emotional growth, by stating that he no longer hates hugs (he only finds them extremely irritating).