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.
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.
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.
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?!
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 upon by lots of characters, and various events are retold by various points of view, and others are just mentioned briefly. Most of the flashbacks flesh out those events.
This was partially remedied by the Before Watchmen event, 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.
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 Astérix 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.