When Lucifer, in Castiel's body, touches Dean, he touches his right shoulder several times; the true Castiel has always touched Dean's left shoulder, which is the side that bears the burn mark of his hand.
Sam says, "I mean, if James Cameron and his Avatar billions can't find it." He is referring to the seven deep-ocean expeditions that Cameron lead to collect forensic evidence from the wrecks of the Titanic and the Bismarck, and to map deep hydrothermal vents in the Atlantic and Pacific oceans.
When Lucifer is teasing Crowley about his defiance he calls him the once and future king of hell. This is in reference to the book The Once and Future King, by T.H. White about King Arthur, who according to legend would one day return to rule over Britain, just as Crowley hopes to rule over hell once more.
The French song that plays several times throughout the episode is "Non, Je Ne Regrette Rien", which means, "No, I Regret Nothing". Although this episode takes place during World War II, the song was written in 1956 and recorded by Edith Piaf in 1959, many years after the end of the war.
This episode is the first time an angel is seen appearing on screen after teleporting, or in this case time travelling. All the other times this has happened on the show, it was out of shot.