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 Brothers 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.
Probably 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 one episode of Sealab 2021 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 didn't see 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, Evil Kinevil 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.
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 Japanese midget apparently does something awesome, but we never see what.
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.
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.
A huge one in "It's About Time". Twilight sets out to return Cerberus(!) to the gates of Tartarus(!!), so that all the evil creatures that are imprisoned there won't break free and destroy Equestria(!!!). And all we get to see is Spike waking up next morning and remarking that Twilight should go on epic adventures more often.
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 the MGM Screwy Squirrel cartoon Happy Go Nutty, the dog chases Screwy Squirrel into a cave. A few seconds of absolute darkness, with a few cartoony sound effects, then Screwy lights a match and tells the audience, "Sure was a funny gag. Too bad you couldn't see it."
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.
