The demon Crowley reveals something important to Sam and Dean on the episode "The Devil You Know." He tells them how he's been able to track Sam and Dean.
Bobby asks in this promo: How are you gonna control the Devil when you can't control yourself? Check it out now.

Supernatural Season 5 Episode 20 Quotes

Sam: What if you guys lead the devil to the edge, and I jump in?
Bobby: Sam...
Sam: It'll be just like when you turned the knife around on yourself. One action, just one leap.
Bobby: Are you idgits trying to kill me? We just got done talking your brother off the ledge, now you're lining up to say yes... This isn't an option, Sam.

Dean: [wearing a surgical mask] Hey, I looked like the King of Pop.
Sam: [Rolls eyes]
Dean: Too soon? Too soon.