When looking at the painting together, Dean (Jensen Ackles) calls Sam (Jared Padalecki) "Jared" instead of Sam. He has actually admitted to this on a panel an a convention and said that the editing team missed it, so it is in the episode as the final cut.
Sam references the book The Da Vinci Code, written by Dan Brown, when Dean suggests there's possible hidden clues in the cursed painting. Dean doesn't understand what Sam means and says he's "still waiting for the movie on that one." The Da Vinci Code (2006) premiered in theaters a month after this episode aired.
One of the cars featured at the auction house has the registration "The Krip". Eric Kripke is the creator of the show.
When Sarah says that the family portrait is in the American Primitive art style, Sam corrects her by saying that the style is more Grant Wood. Grant Wood is an American painter whose most famous work is the "American Gothic" (1930).
Sam & Dean are seen going into their motel room which has a 1970's black & silver decorating theme.
Disco music is being played in the background and when the door is closed you can see the do not disturb sign which bears a silhouette of a male dancer in a pose made famous by John Travolta in Saturday Night Fever (1977) and the motel name of The Boogie Inn.