Some of my favorite moments in film and TV are the little, blink-and-you-missed-it "throwaway" lines that good writers know how to deploy effectively to inject some humor or set a mood.
Heard just now on "Falling Skies":
"So you were in a gang...what sort? Bloods? Crips? ...Slytherin?"
Not sure if this counts but hey, it's funny. I've just been re-watching Spin City for the first time in years and I completely forgot about the 'second' problem they had to damage control in the pilot. It is played as a throw-away line, but it dictates the entire remainder of the episode. Look away now if you want to hunt it down and enjoy the gag without it being spoiled:
The whole staff have just watched the Mayor give the correct "spontaneous" answers that James wrote for him in response to a sanitation worker strike on live TV. Everyone's breathing a sigh of relief and the camera starts to pan away from the TV when, in the background, you hear a reporter ask the Mayor if he'll be attending the Gay Pride rally on the weekend. His response...
"What, are you drunk?"
It blind-sided me and I actually woke my son up because I was laughing so hard. The follow up look on Michael J Fox' face then proceeded to almost kill me.
Malcolm: He’s making Paul Remington a Cabinet Minister. Remtard Remington. I mean the guy is an epic fuck-up. He’s so dense that light bends around him.
[on the phone to a colleague about how busy he is] Malcolm: I've got more on my plate than a spinster at a wedding. That wasn't a reference to your daughter by the way, Andrew. [later in the episode, on the phone again] Malcolm: Doug Hayes is a massive abortion. Again, not a reference to your daughter.
Malcolm: Here he is! Cock like the Pink Panther's tail. Doug Hayes: I'm afraid I turned it down, Malcolm. Malcolm: Do you know 90% of household dust is made of dead human skin? That's what you are to me.
Malcolm: You hit the phones, right. I'll be with you in two shakes of a crying baby.