Five Second Foreshadowing

The audience is informed of some fact X, which is both shocking and important. There's a bomb in the briefcase, or Fran is The Mole, or whatever. This is often communicated via a character who has just realized X, and says it out loud (thus informing the audience). Another alternative is to use a fact-revealing flashback. In any case, the audience very suddenly realizes X. Less than five seconds later, before anyone has a chance for a meaningful reaction (such as disarming the bomb), X becomes incredibly relevant. The bomb goes off, or Fran shoots Alice, or whatever.

The purpose of this trope is to allow something shocking to happen without confusing the audience. If Alice flips a lightswitch and suddenly her whole house explodes, the audience might be left wondering what happened. If the writers want to avoid that feeling, they employ this trope. Thus, Bob figures it out beforehand and says something like "Carol put a bomb in Alice's house!", or perhaps we're shown a flashback of Carol planting the bomb. In either case, the audience is informed and confusion is avoided. Of course, if we're informed about the bomb several minutes before the event, it won't be as shocking, and in Bob's case it leaves the writers with the trouble of explaining how Bob knew about the bomb, but didn't manage to do anything about it before it went off. Thus, we're informed of the truth, and the truth becomes relevant soon afterwards.

Technically, this trope should only apply when the audience (not just a character) learns something new. If the audience already knows what's up and it's only the characters who suddenly realize something, that's a different trope. Also, if the bomb goes off and the audience is only told after the fact that Bob had it all figured out five seconds previously, that doesn't count either.

It's perfectly fine if there are hints about X scattered around the story, just so long as the audience is not intended to really get it until a critical moment, just before X causes something drastic.

Fanfic

 * "Hey, guys, what's a gas main?"
 * For clarity, the following is said by a monster, leading to Ranma realizing that not only is the fake alter packed with about four times as much explosives as they need due to Poor Communication Kills, the whole shebang is right next to said gas main. Cue Oh Crap.

Film

 * Death Proof: When Pam realizes that Stuntman Mike isn't actually going to give her a ride home, there's a super close up of her eyes. One second before Stuntman Mike hits the gas and speeds off... With Pam locked in the passenger side.
 * The Godfather: Michael realizes something's fishy about the car, and quickly tries to warn his Sicilian wife about it, but shortly thereafter she starts the car and explodes.
 * The Godfather Part II: Michael realizes that the drapes are open a second before his room is hit with a hail of gunfire from outside.
 * Ninja Assassin: Raizo has been captured and taken to the secret ninja temple, but secretly he has a GPS tracking device in his gut, alerting the good guys to the temple's location. The ninjas discover the device, and right after that the temple is attacked.
 * Paycheck: The bad guy stands in front of the machine that shows the future, but only sees his own back, as he is futilely trying to get away from the exploding machine. Soon afterwards he realizes that this means the machine will explode, and futilely tries to get away from it.
 * Star Trek Generations: When Dr. Soran checks on his sun-killer missile after Picard messes with it, the viewscreen says that the missile's locking clamps are still engaged. Soran gets an Oh Crap look on his face as he and the audience realize that something bad is going to happen when the missile tries to launch. Then the missile explodes, killing him.
 * Star Trek: Nemesis: The Enterprise is being stalked by a cloaked ship, so it's hoping to rendezvous with the fleet for protection. On the way, they enter an area of space where long-range communications don't work. Data and Picard realize that this would be a perfect place for the stalker ship to attack them. After that thought, they get attacked.
 * The Dark Knight: "Is that a....phone?"
 * Pirates of the Caribbean: At World's End: "If he's not your man, and he's not our man...whose man is he?"
 * Richie Rich, the movie: Richard uses his smell detector to sneak a few of his gifts on their plane...then it sniffs out "trinitrotoluene." He brings that gift to the cockpit, where his wife figures out it's TNT...he opens the package. "My God, Regina, it's a bomb!" He does manage to get it out of the plane in that one second, but the resulting Kaboom! plants them in the ocean.
 * Tangled: A tiny unicorn figurine appears just before the ruffians show up to rescue Eugene.
 * The Incredibles, had Kari calling Mrs. Incredible about how the replacement sitter had shown up to take care of little Jack-Jack. "I didn't order a replacement sitter!"
 * Home Alone 2: "Now why would anybody soak a rope in kerosene?"

Literature

 * In one of the Robotech novels, "End of the Circle," the planet Haydon IV turns into a Giant robot. On its hands are two massive tuning-fork like prongs. The Sentinal Alliance decides to attack the planet-bot and seems to be doing well, but the planet isn't taking much damage due to pin-point barrier systems and anti-air batteries. It seems normal, except for power buildup in the prongs. Suddenly, Exedore realizes that the two technologies (transformation and pin-point barriers) were both used in the original SDF-1, and that the tuning forks are near-exact copies of the ship's Wave Motion Gun. He starts to call out a warning, but the guns fire, nearly destroying the fleet flagship utterly.

Live Action TV

 * In the Star Trek: Voyager episode "Death Wish," A member of the Q continuum wishes to die ("Quinn," for ease's sake). After screwing up and vanishing every male on Voyager, and pressed by Captain Janeway, he begins to speak to himself, "Humans... humans... Who would have more recent experience with humans?" Cue Q himself.
 * Veronica Mars tended to reveal its season-long villains in this manner.
 * In one Burn Notice episode, Michael is told that another character was killed by a bomb, one second before he opens the door to his own house (which is also rigged to explode).

Video Games
"Tech Sergeant Ybarra: ...a mobile phone? *BOOM*"
 * The flashback approach is taken with The Reveal in the first Knights of the Old Republic. Right before Darth Malak drops the, the game replays every single bit of foreshadowing that had happened earlier in the game.
 * Halo: Reach, after the city mission, Kat declares that the Covenant are beginning glassing procedures. Another Spartan asks, "How close?" Cue massive explosion from the surface bombardment.
 * In the trailer (around 1:50) for the Warhammer 40000 game Dawn of War 2, a Space Marine strikes down an Eldar with his chainsword, proclaiming "This planet is ours, witch!" "No," she gasps, pointing towards the heavens, "This planet... is theirs." Cue Tyranid horde and massive 'Nid bug right behind the Space Marine.
 * In the 2010 Re Boot of Medal of Honor, this happens to Sergeant Peterson's Ranger team as they are about to kick in the door to clear a house, only to hear a mobile phone ring inside. Bonus points as the player knows what this sound means, having seen the same trick in the game's prologue mission, but the characters have no clue what is about to happen.


 * Similar to the Knights of the Old Republic example above, BioShock (series) uses this for The Reveal: The player immediately sees a flashback montage of several scenes in which.

Web Original

 * "Hey, everyone! I found the source of the ticking; it's a pipe bomb!" "Yay!" BOOM

Western Animation

 * A common running joke in Megas XLR was Coop forgetting he had a button, labeled as exactly what he needed at the moment. Another was Kiva warning him that what he or the bad guy was doing would cause something really destructive... and then Coop would trigger that event anyway not five minutes later.
 * In the Ed, Edd 'n' Eddy movie, we finally meet Eddy's elusive older brother. When we do, he asks if anyone knows Eddy's there with him. One minute later...