Done deliberately in Too Much Coffee Man. After studiously avoiding any even vaguely superheroic content in a superhero comic, we finally get an alien coming to Earth and begging the hero for help. The two of them make small talk as they get ready for the adventure ... then the story jumps ahead to show them coming home, since the small talk was the real point.
Squirrel Girl does this all the time. In New Avengers, for example, she takes down a Nazi mech. She then runs into three more. She charges them as they open fire. We next see her at Avengers Mansion, battered and bleeding.
This is a running gag with Squirrel Girl, where she defeats someone like Thanos and Galactus off-screen while her team is dealing with a considerably smaller threat.
In an ElfQuest miniseries, Rebels, "The worst pile-up in many years" during a futuristic race happens mostly off panel. We just see the aftermath.
In Blue Beetle, Guy Gardner starts a "really cool bar fight." We see what causes the fight, we are told that it is really cool, but the fight itself happens off panel.
In New Krypton final arc "War of the Supermen", Supergirl defeats her old enemy Superwoman for good. It's a brutal, one-sided beatdown... which happens completely off panel.
At the end of Preacher, the Saint of Killers takes on the entire Heavenly Host when they get in his way. All we see of it is him standing there surrounded by angel corpses awaiting God's return. So that he can shoot him. Yeah. And even that happens off panel!
Parodied in one issue of The Simpsons comic, when Krusty is pitching his own spy TV show. In the climax of the pilot episode, Krusty's character tells his secretary all of the things he did to stop the Big Bad. (Krusty tells the irate studio heads that he couldn't actually show the action on-screen because he blew the show's budget on one scene involving helicopter shoes.)
Runaways doesn't show Karolina and Xavin's wedding, or the Skrulls and Majesdanians blowing up each other's planets shortly after. Instead of extraterrestrial lesbian weddings, intergalactic war and explosions, we get the rest of the Runaways fighting with each other and falling to pieces.
This was probably intentional, as the entire point of Xavin's introduction was to create a pretense to remove Karolina from the series because Marvel dreaded the wrath of the Moral Guardians who might object to having a open lesbian character in a series that was ostensibly aimed at teenagers. Actually showing Xavin and Karolina's courtship and nuptials would have defeated the purpose.
Similarly, we don't see exactly how Nico escaped from the Witchbreaker during "Dead End Kids", presumably because Marvel was eager to hurry up and close that arc so that the kids could be used for a Secret Invasion crossover with the Young Avengers.
Hellblazer: Seen in the Hard Times story arc. During John Constantine's days incarcerated in a penitentiary, a gang of inmates decided to rape the Englishman while he was taking a shower. Unfortunately John isn't too keen in getting butt-plugged. Although readers never saw what was going on inside the shower room, John seemingly saves his ass by cursing the would be rapists with catatonia. Apparently it was too awesome to be seen.
Frau Totenkinder and Baba Yaga's duel in FABLES. All we saw were lightning strikes, silhouettes of monsters, and King Cole scared shitless, and it still proved to be the best battle yet, and one of the sole reasons for Totenkinder's badass rep.
In the Strontium Dog story "The Life and Death of Johnny Alpha", Middenface and Precious have to break Feral out of a high-security prison where, for the past three months, he has been force-fed in order to be fat enough for a ritual sacrifice. This certainly sounds like an awesome action scene, and indeed the cover implies that this is what the strip focuses on... but instead, as soon as Feral is loose, he passes out, and we cut to a spaceship where Middenface is injured, and remarks that the escape wasn't easy. It's unclear if the sequence was skipped in order to keep the plot going or if Wagner and Ezquerra just couldn't figure out how to show it, but either way it went down poorly.
Justice League: Cry for Justice has Green Lantern and Green Arrow defeating an army of super-villains single-handed and off-panel. The comic jumps from them leaping into battle to another scene, then back to them after the battle is over. And in the final issue, Green Arrow brags about how he and the Jack Bauer League were able to shut down a notorious Somali pirate who is never seen or mentioned before that moment. Sure would've been nice to actually see them doing those things...
Grant Morrison has some issues with endings, but the way the "World War III" arc of his JLA run ended really takes the cake. Every person on Earth gets powers (including Oracle, who, you'll remember, has been paralyzed from the waist down for years.) They join the angelic choir in an assault on a horrifying Eldritch Abomination-style-thing. We see the Earth's population and the angels going into space for one two-page thread... and then we never see any of the fight. Okay, so thematically it was supposed to be about Superman's fight against Mageddon, but how can you tease such a gigantic fight and not show it?!
He'd done it before at the start of his run. When the JLA escapes and goes on the attack against the (disguised as superhumans) White Martians, we see at least part of the one-on-one battles most of them have. Except for Batman, who shows up dragging three unconscious White Martians behind him.
The Thanos Imperative uses this for effect. When the Cancerverse unleashes their Galactus Engine on the normal universe, it is shown to simply sit there doing nothing. Silver Surfer explains that the battle the Engine is involved with exists at a conceptual level (literally different abstract concepts trying to kill each other) so the fighting is impossible for mortals to see, only the consequences. Just as Nova is complaining about not being able to tell what is going on, one of the abstracts on their side explodes.
In Watchmen, all of the plot, flashbacks aside, is set after the Super Registration Act, so a lot of the crime-fighting of the Minutemen (and the individual characters, after the Minutemen disbanded) is actually depicted offscreen. Some of the fights and acts are talked about by lots of characters, various events are retold from various points of view, and others are just mentioned briefly. Most of the flashbacks flesh out those events.
This was partially remedied by Before Watchmen, which fleshed out many of the events that were only alluded to in the original.
In Transformers Spotlight: Cliffjumper. When the Decepticons killed a humanoid female he befriended, he picks up his guns and kills all seven of them. It isn't shown how he did it, but Cliff hammers his friends tombstone with one of the Cons' heads.
A beautifully understated one from The Sandman, in which Lucien, Dream's mild-mannered, polite, and determinedly non-threatening librarian, mentions that Dream's prisoners — all the dark monsters too horrific or dangerous to serve as nightmares — have escaped, then adds:
"A couple of them took refuge in the library. I... dealt with them."
The Wolverine/Lobo fight in Marvel Vs DC took place entirely behind a bar. Most likely because there was no plausible way the writers could think of to have Wolverine (who, at the time, had been stripped of his adamantium skeleton and claws) beat a Superman-class powerhouse like Lobo. (It's later revealed that Professor Xavier paid him to take a dive; Every Man Has His Price, even Lobo.)
Don Rosa deliberately did this as example of how American legends are exaggerated over time. A later tie-in story has Scrooge telling Casey Coot that the whole incident wasn't nearly as awesome as stated, and that the townspeople make it more ridiculous every time they tell it.
In My Little Pony: Friendship is Magic (IDW) Celestia (aided by Spike) has an epic fight defending Canterlot against a horde of giant Cocktrices. The bonus story shows that it's a really, really hard battle, and Spike's aid is invaluable against the giant foes.
Fantomex gets one in the X-Men story arc "Nation X". A group of vicious mutant-hunting monsters known as Predator X attack the X-Men island of Utopia, each one requiring an entire team to narrowly defeat. One of them escapes to New York, and the X-Men track it down...only to find that Fantomex has already killed it single-handedly. Without a scratch on him. In a sewer, without even getting his nice white costume dirty.
Done quite frequently in Asterix to mix up the slapstick a little:
In one story, Obelix is really into collecting Roman helmets as trophies, so we watch him wander away from a fight to stack his helmets up, while the fight rages on.
In Asterix in Corsica we cut from the Corsicans versus the Roman Army to watch three old men discussing the impenetrable, boring family trees of the Corsican warriors, such as whose sister married whose nephew.
When Asterix sets Obelix loose in a Roman fort because he hasn't had anything to do in the story, we watch the chaos from a decent distance, where we can't make out any detail — only Asterix relaxing on a grassy hillside.
The Roman Agent shows the most epic battle in the history of the village in the style of a history book illustration where it's far too zoomed-out for us to clearly see what's happening. This also happens in-universe when the two Non Action Guys are shown waiting in the middle of the village, one of whom is asking the other what's going on.
During Chris Claremont's run on Fantastic Four, Susan Richards faces off against Absorbing Man, She-Hulk and Ben Grimm(the latter two being Brainwashed and Crazy at the time) for threatening Valeria, her time-lost, alternate-universe daughter (it's complicated). Reed Richards urges Johnny Storm, Valeria and a visiting Spider-Man to stand back and let Susan deal with the three powerhouses, seeing this fight as "therapy" for her. We never see the resulting fight, we just read a few random sound-effects and see reaction shots of Reed ("An excellent synthesis of form and function."), Johnny("Oh, that's gotta hurt!"), Valeria("Hit 'im again, Mom!") and Spider-Man("I never, never, never want to make her mad at me!").
An issue of Fantastic Four during John Byrne's run starts with the team returning from a cosmic adventure, which we never actually see. When She-Hulk (who was subbing for the Thing at the time) comments that their adventure would provide some entertaining stories for their comic book, the Human Torch comments, "I dunno, Jen. Last I heard there's a theory among the publishing companies that 'Cosmic Doesn't Sell'." This leads to a brief Author Filibuster where Johnny Storm, speaking for John Byrne, advises the readers to contact Marvel Comics and tell them that they want Cosmic stories.
In Tintin in the Land of Black Gold Tintin is cornered by the villainous Dr. Müller's henchmen, when his friend Captain Haddock, who has been absent through most of the story, pulls a Big Damn Heroes moment and saves him. We never see how Haddock found out where Tintin was or his battle with Müller and his men.note This was done due to that particular story's Troubled Production History. Herge had already published about a third of Black Gold, when the newspaper he was working for was closed down by the Germans in 1940. He then started publishing a new Tintin story in another newspaper, where he introduced the character of Captain Haddock. When he took up Black Gold again after the war, Haddock had become such an established part of the series that it would be impossible not to include him in the story. Herge lampshades this by never even explaining why Haddock suddenly shows up: Haddock is always interrupted, when he tries to tell his story and only manages to say that it is "simple and complicated at the same time"
In the 1980s, most of the X-Men faked their deaths during the "Fall Of The Mutants" storyline. The survivors formed a new team, Excalibur. The "dead" X-Men went around doing good in secret, until it eventually became clear to the world that they weren't dead. We never saw the moment when the members of Excalibur learned their old friends weren't dead, and instead learned, in the letter column, that they had found out and been in contact over the phone. Given that Chris Claremont was writing both titles (he quit Excalibur after the point that the New Mutants, X-Factor, and others all knew the X-Men were alive), one wonders what happened that such an emotional moment was left off-screen.
In Spider-Verse, the massive battle between Leopardon and The Inheritor known as Solus is never seen after the initial blows and we only see Takuya joining the other Spiders in defeat, telling them that Leopardon gave him a chance to escape. This ended up leading to some fans believing that Leopardon got Worfed without realizing that the first tokusatsu giant robot battled a Physical God to a standstill and forced him to lose that power just to win.
This is a mainstay trope of many Astro City stories; since the focus is on the emotional or personal growth of the characters, earth-shattering events are often relegated to mere background color.
"Everyday Life" has the First Family repeatedly tearing through armies of Mooks in search of their missing daughter, yet each battle is relegated to one or two frames, to better focus on her efforts to learn hopscotch.
"Confession" has a worldwide alien invasion with dozens of heroes against an army of shape-shifting extraterrestrials, with a dozen panels devoted to the actual battles themselves.
Issue 6 has The Maestro versus The Grandmaster. It manages to last four minutes, but we only see the end result: The big green guy complaining about how it wasn't fun, while the Grandmaster lies on the floor waiting for his bones to knit back together.
The Ultimates: During Ultimates 2, the European Union Super Soldier Initiative sneak into Liberator-occupied territory and bust out the other heroes, all on their own. And almost entirely off-page.