The amount and position of streamers on Raj changes between shots.
Howard claims that he had never had a threesome, but in the episode The Stag Convergence (2012) Raj drinks a toast to Howard at his stag party and tells the group of their encounter with a woman dressed as Sailor Moon at Comic-Con: "The only threesome I've ever had in my life, and I'm proud to say it was with this man right here."
Correction: The threesome could have happened after this episode but before the later one.
Correction: The threesome could have happened after this episode but before the later one.
When Penny goes across the hall to propose holding a surprise party for Leonard, she knocks on the door and waits for Sheldon to answer it. In previous episodes she knocks on the door and just walks in.