You Know What That Means

Whenever one or more characters recognizes something ominous, they apparently feel the need to point it out by saying something like "And you know what that means!" or "Do you know what this means?" This will almost always be followed up by an explanation of what it means, either apropos of nothing or because one member of the group really doesn't know what it means. In a rare few cases, it instead leads to a Cryptic Conversation, and the audience doesn't find out what "that" means until later.

Literature

 * Used in Lords and Ladies by Granny Weatherwax and Nanny Ogg, regarding the crop circles appearing across Lancre. Magrat is the one who doesn't know what it means (that the elves are trying to invade), and the other two don't enlighten her because they think Magrat won't understand that the return of the elves is a bad thing.

Video Games
"Agent #1: Don't be a stiff. You know what it means?
 * In the first Destroy All Humans!! game, two Majestic agents are surveying the damage done by Crypto during the first level.

Agent #2: Absolutely... what?"

Western Animation
"Sam: Do you realize what this means?!
 * In the Adventures of Sam and Max Freelance Police episode "Big Trouble at the Earth's Core", Sam notices the temperature of the Earth's core is reaching critical levels.

Max: No, but let me guess!"

"Twilight: Spike, do you know what this means?
 * Used in the first episode of My Little Pony Friendship Is Magic, after Twilight reads up on the legend of Nightmare Moon:

Spike: ...No?"

"Pinkie Pie: It's my tail! It's my tail! It's a-twitchin' twitchin'! And you know what that means!
 * Later used in "Feeling Pinkie Keen":

Twilight Sparkle: Actually, Pinkie, I haven't the slightest idea."