While they were filming "Teenage Dream" (the breakup version), Chris (Kurt) couldn't stop crying and had to leave the set for a while and when he came back Darren (Blaine) grabbed his face and told him that "If you cry, I cry"
During the song "Barely Breathing" when Blaine is singing to Kurt in the auditorium, Kurt is wearing the exact same outfit he wore when he first met Blaine in 2x06
According to internet rumors, "Mine" by Taylor Swift was written about a flirtation she had with Cory Monteith, who plays Finn.
Heather Morris (Brittany) has said that her tears during Naya Rivera's (Santana) performance of "Mine" were real.
Duncan Sheik wrote the music for "Spring Awakening," which Lea Michele was in on Broadway, as well as "Barely Breathing," which Finn and Blaine sing here.