The "Best Friends Forever" episode of South Park openly revels in this trope, as the ending of the episode is essentially a huge battle between the forces of heaven and hell - which is not seen at all. However, Heaven's force commander describes it, along with wishing he had a camcorder, and calling it "...like ten times bigger...!" than the final battle in the Lord of the Rings movie. Word of God from Matt and Trey said they ran out of time to do an extended battle sequence, so they had to just have one of the angels calling out the action.
Winx Club had one in the first season. There was this big deal about Cloud Tower (witches) and Alfea (fairies) working together to fight off the Trix, since their students are total polar opposites. Come time to show the battle, all they show are the fairies. Even in a scene where the Cloud Tower principal specifically calls for the witches to combine their powers with the fairies, they still only show the fairies.
This trope is intentionally lampooned in The Venture Bros. episode "Escape to the House of Mummies, Part II". The Venture family escape a deadly cultist trap in some ancient ruin but Dr Venture is the only one able to leave the pyramid. While he manages to go back home ("to get help") he forgets he's supposed to go back to get the rest of the family because he is distracted into making a bet with his neighbor (for bragging rights). Meanwhile, the abandoned heroes have somehow made it safely out of captivity and are now accompanied by the Perfect Man, Caligula, Edgar Allan Poe, a time-lapsed Brock Sampson duplicate and a friendly mummy. They are now ready to carry out the final and climactic ultimate badass battle against the Cult of Osiris...but then we simply cut back to the REAL story which is Dr Venture and his buddies talking about the bet and then going for some brews, oblivious that they had forgotten to do something.
Iroh's escape from the Fire Nation prison in Avatar: The Last Airbender. He pulls his Obfuscating Stupidity whenever anyone's watching, while training determinedly in private. When Zuko finally makes his last Heel-Face Turn and goes to rescue Iroh, he's long gone, having trashed everyone and everything in his path. All we find out is that he was "like a One-Man Army."
Another one to do with Mai and Ty Lee. In the episode "The Drill", an earthbender team is sent to handle the drill. Mai and Ty Lee goes out to stop them. Ty gets a CMOA here- but she only takes out about eight of the 24 benders. Mai isn't even seen, and the 16 are treated like they were never even there. In short, Mai just defeated 16 earthbenders on her own, offscreen. It's probably this because Mai had to kill them with her knives, and on-screen death is just a very sensitive topic to get into in Western Animation.
Invader Zim parodies this trope at the end of "GIR Goes Crazy and Stuff," when the (literally) squid-brained cop thanks Zim for taking him back to the ocean, and for the intergalactic space battle that occurred on the way there.
The trope also happens in "Tak the Hideous New Girl," coming back from a commercial break:
Zim:: What a horrible adventure with that Ham Demon! But I'm still no closer to solving this Tak thing.
Happens a third time in "Gaz Taster of Pork", this time with mentions of a missile-firing squid, and the aftermath involving Dib and Gaz driving a beat-up car while wearing beaver costumes.
The show is also canceled right before the episodes "Ten Minutes to Doom" or "The Trial" could be released and, judging by the scripts, many fans think that these episodes would have been quite awesome. Also in the commentary to "A Room with a Moose" a commenter states that there was going to be a huge space battle between Dib and Zim to create a climatic ending to the show but they wasted the budget on the 3D walnuts for this episode.
Lampshaded in the Sealab 2021 episode "Neptunadi", where Tornado Shanks has a massive off-screen battle with a Kraken while the audience is only shown the other characters' reactions. Quinn then says "Anyone who missed that should just go kill themselves right now!"
Parodied in the Cartoon Network Super Bowl spoof Big Game XXVIII. Much of the pre-game and first half are spent hyping this huge halftime show featuring Tom Jones, Carol Channing, Evel Knievil and a dancing monkey. Due to a camera snafu, we only see Scooby-Doo's Fred and Space Ghost's Moltar bantering in the control truck.
The episode "Jake the Dog" features an example that is at least as horrifying as it is awesome. Alternate Finn has transformed into the new Ice King, with little of the goofiness and ineffectualness of the original, and is about to attack Alternate Jake, who has just mutated into the Lich. We don't get to see the ensuing fight, as Prismo's TV, through which the audience is seeing these events, goes out just as it's about to start.
The episode "Earth and Water" has Flame Princess and Cinnamon Bun staging a coup against the Flame King and imprisoning him in the very lamp that FP was sealed in when she first appeared in "Incendium".
And let's not forget in Billy's Song, when he "threw the Lich King down". It was never seen how he did it but it would be implied to have been an awesome battle against the greatest evil that ever existed and the greatest hero ever born.
Almost every episode of Futurama mentions a Noodle Incident which would've been really cool to see. Of course, if we got to see all of them, it wouldn't be Futurama anymore.
The Simpsons episode "The Wandering Juvie" we are treated to an episode of Itchy and Scratchy where an army of Itchys and an army of Scratchys engaging in a battle parodying Braveheart, just as the action begins it cuts away saying that it's been edited for prison viewing and we cut to the Itchy's celebrating their victory and the spectacular 3D blood and gore from the fight.
And then there's the episode where they bring in the Mafia and the Yakuza. A short Japanese man apparently does something awesome, but we never see what. We only heard a Kiai and the noise of several bodies colliding, and later we see the Yakuza doing crazy handsprings across the garden. Imagine the rest.
Lampshaded by the fact that Homer doesn't see what he does either, and is profoundly disappointed by that fact.
Another episode has Krusty showing an Itchy and Scratchy cartoon where Scratchy finally wins and has Itchy absolutely loaded down with explosives, including sticks of TNT shoved under his eyelids, plastic explosives put on his face in the shape of Abe Lincoln's hat and beard, and two giant missiles pointed directly at his face. And then one of Homer's nerdy friends unplugs the TV so he can plug in his rock tumbler. By the time they get the TV back on, Krusty is declaring that they'll never be allowed to show that cartoon again and burns the film.
Mr. Burns evidently subdued the Loch Ness Monster single-handed. All we see of this is Burns rolling up his sleeves to "do it [him]self", then it cuts to the monster suspended from their helicopter and Homer and Willie expressing their incredulity.
Burns: Yes, I was a little worried when he swallowed me, but well, you know the rest.
A mook battle sequence in the Earthworm Jim episode "The Egg Beater" is interrupted by a newscaster announcing that costs of cartoons are skyrocketing and that producers are seeking substitutes for huge action sequences. The scene then cuts back to Jim, post-battle. A major battle where he fights all of his major villains, however, is shown immediately afterwards and in full.
In the Synchro-VoxMr. Incredible and Pals short, Frozone creates a bridge out of ice off-camera, while Mr. Incredible points out how amazingly this feat demonstrates Frozone's powers. In the commentary, Frozone complains that the fact he created the bridge off-camera prevented viewers from seeing the best part of the short.
In Inspiration Manifestation, we don't get to see Luna and Cadance join Twilight to restore Ponyville; we only hear Twilight saying that it happened.
The titular superhero team of The Avengers: Earth's Mightiest Heroes was founded in the wake of 74 prisoners breaking out of jail. Since the show currently has a schedule consisting of 52 episodes, some of the villains get re-captured in between stories, or during subplots the viewers don't get to see.
In one Phineas and Ferb episode, the titular duo, and their friends, apparently build something really big, which is not seen except for a few unfinished parts because of the episode focusing on Candace being sent to Easter Island because of the cell phone they programmed for her. When she returns, all we see is Phineas saying that it was really big, and Ferb adding that it was hauled to the museum.
In Doof Dynasty, Phineas and Ferb rescue Isabella. In the time it takes for them to get up the booby trapped staircase and slide down the rail, Buford and Baljeet have defeated every single warrior in the tower.
In the King of the Hill episode "A Man Without A Country Club", while practicing for the country club Dale asks "What happens if my tee shot lands on a bird's back and he carries it out of bounds but then is attacked by a larger bird who grabs the ball and drops it in the hole? Is that still a hole in one?, 'cause that's how I'm gonna play it.", and Hank replies "Dangit, Dale, it already happened once, what are the odds of it happening again?".
In The Powerpuff Girls episode Him Diddle Riddle, the trio are given the riddle "In the ear of corn, you will find; happiness, joy, and the ties that bind; squirrels store nuts and birds sing songs; but in the cave of eternity, everyone's wrong; on the limb of a tree, there's a monkey who's free; and there he will give you something for me.". Cut two minutes later to the next shot with the girls exhausted. Him, who is holding an ice cream cone, even states "I can't believe you got that right. You got the right flavor and everything!"
An episode of The Grim Adventures of Billy & Mandy is about to go into a massive and very violent battle between mushroom-harvesting dwarves and cookie-making elves, but a Drill Sergeant Nasty (played by R. Lee Ermey) pops onscreen and deems the fight will be too graphic to show to the younger, more sensitive viewers... so he has the scene cut to footage of a cute pink koala while he comments on how epic the battle is. However, when he thinks the battle is about over, he jumps the gun and we catch a glimpse of the mayhem- until the sarge cuts right back to the koala. ("Woah, that was entirely my bad! I misread the signals!") The next time he switches scenes, it really is over.
In Xiaolin Showdown, Jack Spicer becomes trapped in the Yin-Yang World and comes across Hannibal Roy Bean, who happens to be one of his "evil heroes." Hannibal, being a master manipulator, tells Jack that he could be a great villain, but the reason he works so hard at being evil is his insecurity. Jack meekly protests that he is evil, and Hannibal replies If You're True Evil, Let Me Out. For the record, Jack is shown desperate to prove himself as a villain, as well as the resident dupe to the other villains (and sometimes the heroes). Cut to the Xiaolin warriors finding Jack locked in the cage, having apparently fallen for it. Then they run into another Jack, and it's revealed that Hannibal used the Moby Morpher to disguise himself and trick the good guys. Subtle, and it only added another couple minutes to the episode's run time, but it sure says a lot about Jack's character.
In the Recess episode "A Great State Fair", most of the episode focuses on T.J. trying to contact Gus's dad, who forgot to give Gus his permission slip to the fair. One of the things T.J. tried attempting, according to Vince, was making a run for the interstate in a bumper car. This was never seen in the episode.
In the finale of Transformers Prime, Bulkhead, Wheeljack, and Ultra Magnus are groundbridged off of the bridge of the Decepticon warship, and into a room full of what looks the the entire remaining force of vehicons. We don't see them again until near the end of the episode, where they are completely unscathed. What a fight that must have been.
The Freakazoid! episode "Relax-O-Vision" runs on this trope. The Lobe orchestrates a Villain Team-Up and kidnaps Steff, forcing Freakazoid to fight to save her. However, every time some action is about to start, the episode cuts to Relax-O-Vision, inoffensive Stock Footage with pleasant music. After saving Steff, an annoyed Freakazoid decides to beat up the guy who invented Relax-O-Vision... as more Stock Footage rolls.
''Rocko's Modern Life: One episode revolved around the idea of Rocko sending his parents back in Australia a video of his daily life. During one scene, he focused on Spunky, telling him to "do his trick" for the camera. We don't get to see said trick as the camera battery dies out and fades to black, but we do get to here Rocko and Heifer's utter amazement at whatever it was.
Teen Titans: As the Titans and their allies are captured in a series of coordinated attacks, The Brain gloats over Robin. Cut to Beast Boy, who's still free. How? We don't know; we only see him standing amid the smashed up bodies of the robots sent to capture him.
In an episode of Yogi's Treasure Hunt the gang is trapped in a room filling with water, and as Huckleberry Hound yells for help the show fades out to an ad break. When the show returns the gang are safe and sound on the roof of the building, and Huck tells the viewers that they'd have seen an incredible escape "if it weren't for that there commercial interruption" ("Yeah, take Huck's word for it," Snooper adds, "it was a doozy!").
In the Gravity Falls episode "Boss Mabel," Dipper somehow defeats and captures a monster called the Gremloblin, who can show you your worst nightmare if you stare into its eyes. It happens offscreen, but all we see is Dipper heading out to the woods with a ridiculously large spiked mace in hand.