During the scene when Cas starts seeping with leviathan blood actor Misha Collins was violently ill and had to throw up between takes.
When Dean replies that he's fine, Bobby quips, "Course. Just lost one of the best friends you ever had, your brother's in the bell jar...." Bobby is referring to the novel "The Bell Jar" (1963) by Sylvia Plath about a woman's descent into insanity.
Dean introduces himself as Special Agent Ian Anderson. Anderson is the flautist and front man for the band Jethro Tull.
Sam says he can't follow up on a lead with Dean because he's "5150'd". That's the California Welfare and Institutions (WIC) code used for an involuntary psychiatric hold. It is also police code for someone who is crazy.
When Sam says he's not seeing white rabbits he is referencing the film Harvey (1950) where the main character has a possibly imaginary friend named Harvey who is a six foot tall white rabbit