The Lord of the Rings: Happens twice with Aragorn, Legolas and Gimli immediately prior to the Battle of the Pellenor Fields.
The awesome kraken in Pirates of the Caribbean was killed off in the third installment. Offscreen. All we get to see is its beached corpse. That such a mysterious and awe-inspiring creature could be killed so anticlimactically because of Cutler Beckett's desire for order and control represented the death of the fantasy and adventure of the age of the pirates.
In Harry Potter and the Goblet of Fire, the Quidditch World Cup is hyped as a major event throughout the opening scenes, but we only see the beginning and aftermath, in a rather abrupt scene cut.
The Predator franchise ended up doing this in the original and the first sequel. In the first one Billy Sole seems to pull a You Shall Not Pass scene when he stops at a tree trunk and pulls out a machete, only for the battle to never be shown. In the sequel when the voodoo priest who controls the Jamaican gangs ends up alone in a dark alley he pulls out a Sword Cane and challenges the Predator head on. All we see is his severed head.
Steven Spielberg's adaptation of The War of the Worlds had a scene where Tom Cruise's son climbs towards the crest of a hill, behind which is what is most likely the final stand of the armed forces in an all-out battle against the Martian Tripods. Just as he (and the audience, due to the camera angle) are just about to glimpse this spectacle... Tom Cruise tackles him. The majority of the rest of the film involves looking at the ankles of tripods from a dingy basement.
In Star Trek: First Contact: Here, we finally have a big budget movie with proper big budget movie effects. We have the most awesome bad guys the series had come up with in years in a massive throwdown with the Federation fleet. . . and we get to listen to it over the radio while the Enterprise bridge crew stand around and look concerned? Whiskey Tango Foxtrot?!?
Episode I: Most of the podrace was left on the cutting room floor. In the film, we only get to see the edited highlights. The 2006 DVD reinstates some of the lost footage, but the introduction is still edited down, leaving several of the contestants unnamed.
Episode II: In the rough cut, a group of Jedi evade the Trade Federation's starfighters and board the mothership, then proceed to work their way to the control center and destroy it. The battle droids all shut down, as at the end of Episode I, only to reactivate soon after when a countermeasure kicks in. This would have been really cool to see, although the movie is really long already and everything that was left in is pretty crucial.
Episode III: For a good two decades, Star Wars fans had been waiting to see two events: Darth Vader's fall to the dark side and his consequent slaughter of his Jedi brethren. How could that not be one of the Crowning Moments of Awesome of the entire series? By only showing a couple seconds of Anakin's raid on the temple, relegating the bulk of the Jedi-slaying to the clone troopers.
Episode IV: Emperor Palpatine permanently disbands the Imperial Senate (formerly the Galactic Senate), a governing body that plays a large role in the saga up to this point...except we don't get to see it. Not in the original, not in the Special Edition, and not in the 2004 DVD. We don't even get to see Palpatine in the whole film. Or Coruscant.
Also, the scroll at the beginning said that the Rebel Alliance won it's first battle against the Imperials and swiped the plans for the Death Star. What we get to see is a lone ship trying to outrun a Super Star Destroyer with the plans.
Done intentionally in Grindhouse, in the Robert Rodriguez directed Planet Terror: Cherry & Wrey are just getting into a fiery sex scene... when the film suddenly cuts, and the screen says "Missing Reel." When it returns a few seconds later, the restaurant has caught fire, the zombies have broken through their defenses, and the formerly asshole sheriff does an abrupt face-turn to Rey, apologizing to him after he did something apparently awesome in the time gap. Bonus points because the sheriff is also lying on a table bleeding to death because one of his own men accidentally shot him during the missing reel.
In The Matrix Reloaded we are told that a large number of hovercraft are assembling to fight a pitched battle against a large number of Sentinels. The battle itself gets two lines of dialogue in the last two minutes of the film.
This is the point of Reservoir Dogs — in a movie about a heist, we never see the heist, or even any of the planning of it. All we see are immediately before and after, as well as a bit of backstory on a couple of the characters.
Alyson Reed, who plays Ms. Darbus in High School Musical, was a former Broadway actress, even playing Cassie in A Chorus Line. She didn't sing a note. Apparently, a song that featured duet between Ms. Darbus and Coach Bolton fighting was written for the first movie, but it was cut before it was even recorded and didn't even make it on the soundtrack as a bonus track.
Intentionally done in Wet, Hot American Summer, where one of the camp counselors goes to rescue the rafting campers, the camera cuts to a reaction shot of the other counselor exclaiming "Wow! You're doing it! You're really doing it! This is incredible!"
In the direct-to-video giant monster movie Zarkorr The Invader, there is an intense battle between the Air Force and the eponymous beast. Fighter jets attack Zarkorr with napalm, engulfing the entire valley he is in in flames. Once the flames begin to die down, it is revealed that Zarkorr is completely unharmed, and he returns fire with energy beams from his eyes, blowing the jets out of the sky one-by-one. It's a shame this entire conflict takes place as a radio report the main characters listen to in their car.
One of just two scenes still missing from the restored Metropolis is Joh Fredersen fighting Rotwang and kicking his butt.
Tsukue Ryunosuke, the Villain Protagonist of Okamoto Kihachi's The Sword of Doom, is forced to go on the run after killing another samurai in a fencing match. The samurai's younger brother, Hyoma, has spent years tracking down his brother's killer, only to find him by accident. He issues a challenge to Ryunosuke to face him in an duel, a challenge that Ryunosuke accepts...but then backs out on. Even after several scenes of Hyoma training to defeat his rival, even after tracking him down yet again—a year later, and by another lucky accident—the movie ends without the climactic face-off.
In the 2000 film Supernova, a starship captain is sealed in an abandoned mine on a rogue moon. Abandoned by all but the robots, which come in all shapes and sizes. They suddenly spring to life, showing off what look like expensive animatronics. The villain comes on the PA system and informs our hero that the robots are all under his control. Then the point of view changes to the ship, and the next time we see the hero is after he's gotten past all the robots, broken out to the moon's surface, and returned to his ship via some shuttle.
While this is more of a offscreen moment of hilarity, we have The Consultant, a short film set in the Marvel Cinematic Universe. The World Security Council wants to put EmilBlonsky on The Avengers. Knowing that this is a recipe for disaster but unable to deal with it directly, SHIELD sends the most obnoxious person they know to handle negotiations with General Ross for Blonsky's release: Tony Stark. While we do get to see the first minute of Tony's meeting with Ross in a bar, the rest is left up to the viewer's imagination. The end result is that negotiations go up in flames (exactly what SHIELD wanted), Ross tried to get Tony thrown out of the bar, and Tony responded by buying the bar and having it demolished.
A key scene in Captain America: The First Avenger where Steve has to make an impossible leap from one platform onto another as the HYDRA base explodes and crumbles in flames around him. While we do see him attempt to jump, the scene cuts away to the Allied camp, and the implication that Steve had perished in the base. However, we then see Steve returning with all 400 POWs, indicating that he did make the jump.
In Captain America: The Winter Soldier, Steve and Natasha are told that the last remaining Falcon suit is in a heavily guarded vault in Fort Meade. In the next scene, Falcon is wearing said suit while he helps them interrogate an enemy agent. The details of how they stole the suit are never mentioned.
Tremors II has Burt Gummer recounting his encounter with a mass of Shriekers who ambushed him, describing how he "dropped the first wave with semi-auto fire" then crushed the majority underneath the wheels of his giant truck, and finished off the survivors "with a combination of small arms fire and hand-to-hand techniques" before finishing up by saying "I am completely out of ammo. That's never happened to me before!" Arguably, his recounting is almost as good thanks to his actor's brilliant delivery of the entire spiel, leaving the whole thing to our imagination. Watch it here.
In Kingdom of Heaven, the audiences sees the preparations for the Battle of Hattin, before the film cuts away to another location. Once it returns to the Horns, the battle is already over and all we see is its bloody aftermath. Instead, the Siege of Jerusalem serves as the Climactic Battle.
Zig-Zagged in the Icelandic film Astrópía, when the Deep-Immersion Gaming fantasy setting from the main characters' roleplaying game comes back for the duration of the climatic real-life fight scene.
Ocean's Eleven does this with the theft of the "pinch", for comedic purposes.
The Grey builds up to an epic, climactic fight between the hero and the alpha wolf of the pack that has been hunting him the whole movie. He tapes airplane liquor bottles to one hand for a makeshift knuckleduster, tapes a knife to the other hand, and runs at the alpha. Then the movie ends. The Stinger has Ottway and the wolf lying in a heap, both breathing their last.
In Crank High Voltage, we see Chelios enter a run-down building occupied by prostitutes and criminals. Immediately after he enters, hookers start running out screaming, and thugs go flying out the windows, doors, and walls. Presumably Chelios is kicking ass like it's nobody's business inside, but we don't get to see the actual fighting.
The climax of horror movie Creature , with the main character fighting the creature, is done off-screen. To make it worse, there is No Ending.
In Titan A.E., there's a scene with resident Wrench Wench Akima waking up in a Drej escape pod, in the middle of a cell full of soon-to-be-auctioned slaves - most of whom are much larger than her and all of whom are looking down at her with obviously predatory intent. One short scene later, the rest of the crew arrives to rescue her, and finds her sitting on top of a pile of unconscious prisoners, grinning and greeting them with "What took you?"
In Iron Man 2 we never see how Vanko escapes (and kills) the Hammer goons sent to watch him, but according to the scene after, it was damn bloody.
In Pacific Rim, Coyote Tango's battle against Onibaba is heard entirely from Mako's perspective as she hides in an alleyway. In a flashback, no less.
Invoked for the climax of Serenity despite the viewpoint characters being right in the middle of it; the camera is focusing mainly on the eponymous ship as they attempt to dodge through and around the massive Alliance Navy vs Reavers throwdown engineered by Mal. We only see the occasional explosion or chunk of debris hurtling across the screen, and it actually works really well.
The Monuments Men was based on a true story, and they touched on the part where a couple of the Monuments Men accidentally stumbled upon Germany's gold reserves while searching for stolen art, but in the film it's treated as just something that happened and that was kinda cool, wasn't it? In Real Life, though, this accidental discovery did more to end the war than almost any soldiering on the part of any of the Allies: the world was still on the gold standard, and when word got out that there was nothing backing the deutchemark, the Third Reich had no way to continue to fund their war effort, and the Wehrmacht fell apart very quickly after that.
In the BIONICLE movie The Legend Reborn, a gigantic army comprised of fighters, villagers and battle chariots appears to challenge the combined army of the Skrall and Bone Hunters. We never see any of them fight, only the five main characters. After the battle's done, Vastus and Tarix, both legendary fighters, arrive to the scene, claiming that it's a miracle they survived the fight which viewers didn't get to see. The DVD revealed that a lot of the battle got cut, although even the planned scenes wouldn't have shown much more.
In Resident Evil: Afterlife, when the group escapes the prison through the sewers, Luther West is grabbed by the zombies and dragged into the darkness. Everybody sadly concludes that he's dead and moves on. Near the end of the movie, Luther emerges from the sewers, alive and well, and kills the last of the zombies chasing him.
In Attack the Block, an alien enters an elevator that the gangster Hi-Hatz entered just as the doors close. We hear screaming and fighting noises. The elevator comes to another floor and opens. Hi-Hatz steps out, covered in blood but fine, with the alien dead.