Continuity mistake: When Beckett goes to the bathroom to read Castle's book and Castle peers over the top of the cubical to tell her what page the sex scene is on, she is holding the book with her fingers between the pages dividing the book into 2 sections. These 2 sections switch to 3 and then back to 2 between shots.
Trivia: In this episode, Castle says the words "My partner is crazy and may start firing at any moment" in Chinese. Beckett then asks him "Semester abroad?", to which he replies "Nah, a TV show I used to love". This is a reference to Firefly, a TV show starring Nathan Fillion in which characters occasionally spoke in Chinese.
Question: Before this episode, there was a Christmas episode "Secret Santa" and then there was "Significant Others" with Meredith showing up to take care of Alexis. In that episode, Beckett mentions choosing the movie Valentine's Day for a date night, causing her to lose a turn. In this episode, Castle and Beckett are celebrating their first Valentine's Day together. So, are they celebrating Valentine's Day twice? This wasn't too long after the Christmas episode and when Beckett made her movie choice, no snow was on the ground. Is this a mistake or is this really their first Valentine's Day together?