Now You Tell Me

"Monk Gyatso: (appearing in a vision/memory) Remember, Aang, if you ever fall into the water during a storm, never freeze yourself in a block of ice like a complete idiot. Aang: (thinking) Well, I screwed the pooch on that one, didn't I?"

- Avatar: The Abridged Series

A comedy gag where a character gets a warning to not perform a certain act, but just too late for it to be useful. Either disaster has already occurred or it's too late to take the action back, and the disaster is moments away. Sometimes, for extra silliness, the nature of the warning is obscured until the discouraged action is taken, such as a message on the screen that warns you not to read the screen.

Can also be Played for Drama, often creating a very stinging sense of Irony. It can also overlap with Remembered Too Late.

Compare Can't You Read the Sign?.

Anime

 * A rather non-comedic use of this trope (or perhaps a Black Comedy use) shows up in Ranma ½. Genma Saotome doesn't read the next page of the training manual detailing the Neko-ken, which says that the technique is impressive sounding, but non-functional, until after he's already trained his six-year-old son Ranma in it. Oh, and it isn't just useless, it causes permanent mental damage to the trainee, afflicting them with a permanent and deeply rooted phobia of cats.
 * The Slayers uses this when the gang is hunting through a dungeon of some sort. Lina is reading warnings out of a book, and Gourry keeps picking up on the first half of the warning instead of the whole. The first warning is something like "Pressing the button... is something you should not do." For the second, Gourry doesn't press the button, but the warning is something like "Don't walk past without pressing the button."
 * Happens a lot in Shukufuku no Campanella. Salsa will do something, and Ritos will tell her about a certain hazard too late for her to avoid it. Salsa shouts the trope name, which then brings Gravity Is a Harsh Mistress into things.
 * Happens from time to time with Vivi in the Alabasta arc of One Piece, as she forgets to tell the party certain things at critical moments, like not mentioning what Mr. 2 looks like until after he leaves the ship.

Films
"Madeline Ashton: Bottoms up! (drinks potion) Lisle von Rhoman: Now, a warning... Madeline Ashton: NOW a warning?!"
 * Death Becomes Her.


 * Back to The Future: Marty is blown across the room when he tries to hook up to the amplifier. Then the Doc calls and warns Marty not to do that.
 * The film version of Harry Potter and The Goblet of Fire. The owl that delivered Sirius' letter bites Harry on the hand. Then Harry reads the letter's postscript, which says "the bird bites".
 * We Were Soldiers: Moore puts out a call to his helicopter crews that the LZ is hot. Crandall, the lead pilot, receives the message at roughly the same time that bullets start pinging off his Huey's airframe.

Literature

 * The Dresden Files has Harry go to a Red Court party, where Harry meets Thomas, a White Court vamp. After drinks are served, Thomas quickly goes to Harry to tell him to not drink anything, as it's been spiked with Red Court venom. Thomas then looks at Harry's empty glass. "Oh." Harry, showing more sense than most, finds a potted plant and throws up into it. He still doesn't feel well, but the lower dose allows him to get better.
 * The story "The Cold Equations" has a tragic example, when the stowaway learns that the penalty is.

Television
"Blake: Next, clip the blue wire, just above second lug nut. Hawkeye: The blue wire, the blue wire... (clip) Blake: But first..."
 * In the Buffy the Vampire Slayer episode "Fear Itself", while the gang is attempting to stop the demon possessing a Halloween haunted house, Giles reads that "there are two ways of stopping the demon from being fully summoned. Destroying the mark of Gachnar--". Buffy promptly tears up the floorboards that the symbol is painted on. Giles continues "--is not one of them and will in fact bring forth the fear demon itself." Oops.
 * Variation on Angel. Cordelia is snooping around an empty house when she has a vision of being attacked by a big ugly demon. As she recovers from the headache, she turns around and finds herself face to face with the very same. "Real helpful!" * Concussion*
 * On Happy Days there was a note saying "Don't close the garage door, it's broken" posted on the garage door.
 * In MASH, during a Wire Dilemma, while reading from a manual:

"Holly: I had a message, but I've forgotten it. Rimmer: Well, it couldn't have been that important, then. (ship shakes) Holly: That's it, "Look out, an asteroid's about to hit us.""
 * From Red Dwarf


 * In the classic Star Trek: TOS episode "The Trouble With Tribbles", the seemingly uncontrollably reproducing Tribbles have just eaten the entire stock of a space station's supply of genetically engineered grain. Moments later, McCoy enters and shouts "I've got it! All we have to do is stop feeding them! If we stop feeding them, they won't reproduce!" A Tribble-covered Kirk replies "Now he tells me."
 * In Drake and Josh, Helen lets Josh pet her hamster, but tells him too late that it bites.
 * In a Sliders episode where the main characters end up in a world where magic is real. During the final fight, the Big Bad turns into a dragon and starts breathing fire, pinning the sliders and his reluctant assistant behind a table. She point to a sword on the wall and says that this is the only weapon that can kill the sorcerer. Cue Rembrandt running for the sword, only to utterly fail at moving it. He runs back, and she continues that only a Mallory can wield it. Rembrandt is visibly annoyed at the late warning.
 * An episode of Full House has Stephanie taking a pretend drive in Joey's car. Thinking the "R" on the gearshift turns on the radio, she backs the car through the kitchen window. Joey later admits the radio doesn't work.
 * A Muppet Family Christmas has a Running Gag about people entering Mrs. Bear's house being warned to be "Careful of the icy patch" after they've already fallen.
 * In the Star Trek: Enterprise episode "Impulse", the crew finds an abandoned (except it isn't) Vulcan ship in an asteroid belt. After Reid and T'pol board it to investigate, Reid casually mentions that an entire Vulcan crew mysteriously went insane and killed each other while on an expedition to recover this very ship. Uh, yeah, Reid, maybe you could have mentioned that before you actually decided to board the ship, seeing as whatever caused them to go insane might be on it...

Radio
"Eccles: Land ahead! FX: CRASH Eccles: I shoulda said dat sooner, shouldn't I?"
 * The Goon Show episode "Dishonoured Again":

Web Animation

 * Red vs. Blue: Church, having traveled back in time, leaves a recorded message of what not to do and the consequences if the others do it in an alien base. All of it happens right as he's speaking it, prompting Caboose to beg "Stop saying these things! Or at least... say them sooner." To the recording.

Video Games

 * Done quite subtly in Modern Warfare 2.

"Legion: Alert: The energy pulse will be hazardous to unshielded organics. Addendum: The interior of this station is not shielded. Shepard: I really wish you'd told me that before."
 * Of course, you learn this after
 * In Mass Effect, there's a side quest where you scan the Keepers, mysterious insectoid creatures that maintain the space station. As it turns out, If you complete the side quest, you get an email in the sequel that gives you this info two years after it would have been useful. But it does demonstrate that the Council is Too Dumb to Live and It's Up to You.
 * In Legion's loyalty mission in Mass Effect 2 you are given the option to break into the heretic geth mainframe and release a high-energy signal pulse to rewrite them into accepting the beliefs of the true geth.

Western Animation
"Joker: Didn't you get gas! Harley: WE'RE BROKE, remember? What was I supposed to do? Fill the tank, shoot the guy, and drive away?! Joker: Mmhmm. Harley: * pause* NOW ya tell me!"
 * In Tiny Toon Adventures, a boat collides with the shore and Plucky falls out onto the beach. Then Babs calls out, "Land ho!" leading Plucky to grumble, "Now she tells me."
 * In Futurama, Fry is sent to deliver a package to the Emperor of a desert planet. He drinks from the first bottle he sees when he arrives in the throne room, which turns out to be their emperor (the aliens are liquids). Thus, according to the culture's laws, he is appointed the new Emperor for having killed the previous one. The package he originally came to deliver is now for him so he opens it and finds a sign reading "don't drink the emperor", which he hangs next to dozens of the same sign in various alien languages.
 * One of the Road Runner vs. Coyote cartoons sees Wile E. Coyote feeding the Road Runner a bunch of Acme Earthquake Pills. When this doesn't do anything to the Road Runner, Coyote swallows the rest of the pills in disgust. Only then does he notice the small print on the bottle: "Caution: Not Effective On Road Runners". At which point the Earthquake Pills start to take effect on Coyote with (of course) hilariously disastrous consequences.
 * In The New Batman Adventures episode Joker's Millions, Harley delivers this great line:


 * Although not seen in the actual show, as the page quote illustrates, it was used quite effectively in Avatar: The Abridged Series where Gyastso warns Aang not to freeze himself in a block of ice.

Real Life

 * Shrink wrap contracts, contracts that can only be read after opening the packaging of their product. Of course, if you decide not to agree to the contract, you can't return the product, because you just opened its packaging.
 * These contracts are generally held to be unenforceable in the EU; in the US the issue is still debated. Either way, most places will still let you return the product if you disagree with the contract.
 * Of course it contracts. If it didn't, it wouldn't be shrink wrap, now would it? Rimshot
 * Truth in Television or Urban Legend? A zoo allegedly had a sign on its monkey enclosure reading, "Warning: These monkeys steal glasses." Of course, this mostly happened whenever anyone wearing glasses leaned forward to read the sign....
 * The warning about the Japanese Pearl Harbor attack was telegrammed through civilian channels... so it showed up a day late. So here we are with a harbor full of sunken steel from the day before and "GUESS WHAT THE JAPANESE ARE COMING".
 * Originally, the Japanese planned to have their ambassador deliver a declaration that they considered peace negotiations to be over, right before the attack. However, the declaration was 5000 words long, so it took too long to transmit and write down. Then the ambassador got stuck in traffic....
 * This was shown in the film Pearl Harbor. The Naval Intelligence officer reaches the conclusion that the Japanese attack is imminent, and it reaches the President a few hours after it already occurred.