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 took 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 started a "really cool bar fight." We see what caused the fight, we are told that it was really cool, but the fight itself happens off panel.
At the end of Preacher, the Saint of Killers took on the ENTIRE Heavenly Host when they got 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 where Krusty is pitching his own spy TV show. The climax of the pilot episode has Krusty's character telling 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 extra-terrestrial lesbian weddings, intergalactic war and explosions, we get the rest of the Runaways whining at each other and falling to pieces.
In the Strontium Dog story "The Life and Death of Johnny Alpha", Middenface and Precious have to burst 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 implied this was what the strip would focus 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 was 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 how the "World War III" arc of his JLA run ended really took the cake. Every person on Earth gets powers (including Oracle whom, if 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 show 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 Crimebusters is actually depicted offscreen, but 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 a moment. Most of the flashbacks flesh out those events.
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 wasn't shown how he did it, but Cliff was hammering 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 with the intent of this incident being an example of American legends and how they 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) had an epic fight defending Canterlot against a horde of giant Coacktrices. The bonus story shows it was a really, really hard battle, and Spike's aid was invaluable against the giant foes.
Celestia also mentions a giant marshmallow pony wreaking Manehattan and the rest of the conversation implies she had to take care of many other threats as well.
