In The Fellowship of The Ring, Merry and Pippin can often be seen goofing off in the backgrounds of otherwise serious scenes.
In The Two Towers, after the big flood we can see an Ent extinguishing his burning head in the water. Unusual in that he's actually in the foreground, but it's still easy to miss.
In Return of The King, Gandalf wryly laments the tragic end of Denethor, the Steward of Gondor, while directly behind him, a maddened Denethor (who has gone mad from grief and guilt) — who is on fire — runs screaming in slow-motion for a long distance before finally leaping straight off the edge of a high cliff. Doubles as Narm (especially Narm Charm) and Meaningful Background Event. The supposed solemnity of the moment is very much not helped by the fact that Gandalf says "So passes Denethor, son of Ecthelion" just as the burning Denethor, well, passes Gandalf and a gaggle of guards as he runs out screaming, making it seem like Gandalf's tribute is an archaic way of saying "heads up, guy on fire coming through!"
Also from the third, as Aragorn is taking a breather at the end of the battle near Minas Tirith, we see one of the giant elephant creatures still running around behind him, looking like it might get a little too close to Aragorn if he's not careful.. and then the ghost army that Aragorn enlisted the help of earlier in the film overruns and overwhelms it until it falls over.
In The Breakfast Club, when John Bender has a "bender moment" and throws his knife into a chair, and gives a long-winded mean-spirited speech, Allison (weird girl) grabs the knife while he's talking.
In Don Juan DeMarco, the opening scene shows Don Juan, wearing a mask, going up to a random woman in a hotel restaurant and romantically seducing her, then taking his leave just before his husband arrives. Later in the film, Don Juan goes into the restaurant without his mask. In the background is the woman, looking around longingly.
In the MTV cult classic Welcome To Joe's Apartment, when the insane artist Walter Mud talks with Joe about his avant-garde experiment to create a new race of "periwinkle" (which involves dumping cans of paint on random passer-by in front of Joe's apartment building), a guy in a three-piece suit and briefcase is seen walking away in the background, his entire head and shoulders dripping in pink paint.
In The Wild World of Batwoman, one long scene of exposition is punctuated by a couple of girls in the background, wrestling over a horseshoe.
In GoldenEye as Q gives James Bond his latest gadgets, we see a hapless lab assistant try to make a call from a phone booth situated in the lab, only to get squashed against the glass by an airbag that explodes from within the booth. For bonus points, we briefly see the phone booth being wheeled off a minute later, with the whimpering assistant still trapped inside.
Played with in Scary Movie 3. The news channel where the heroine works only cares about showing sexy, risque, or obscene things, so they intentionally shoot serious news with provocative things going on in the background.
Any movie by Jim Abrahams and the Zucker brothers. Their movies are practically made of this trope.
Airplane!: "The white zone is for immediate loading and unloading of passengers only." And that's just the first example.
Hot Shots! and its sequel both do a ton of these. We have troop drills involving can-can dancing, four ex-presidents having the crap beat out of them, monks taking turns to impress the first girl they have seen in generations and a reference to the governor of Nebraska making a public apology for his state being so flat. Just for a start.
Top Secret! has a scene where two characters are trying to have an exposition-filled conversation in a pizza place. The conversation quickly gets overtaken by two people struggling with an incredibly cheesy pizza in the background.
Top Secret! has another when Nick, having been told a jacket and tie are required to enter the club, is led away. A foreground conversation then ensues between two minor characters; in the background Nick is seen being measured and fitted with a custom-made suit.
An American Carol features some, one involving zombie lawyers. Given the film's clear stance, it's actually not that surprising that they threw in a background scene with them chasing a couple ambulances.
The Men In Black movies, most memorably the scene (pictured above) with a Human Alien couple where K talks to the man while J struggles to deliver the woman's baby ("Did anything about that seem unusual to you?"). Also, Zed neuralyzing the rejected recruits as J and K walk by.
An easy-to-miss one in Back to the Future because the camera goes slightly out of focus in the background: when Doc Brown is asked for a permit for his "weather equipment", the camera pans downward to focus on Marty putting the letter in the Doc's coat pocket. Meanwhile, in the background, the Doc, retrieving his "permit", is pulling some cash out of his wallet and handing it to the police officer.
In 10 Things I Hate About You, we see one of the main characters in the archery range. Just as she is firing her bow, someone else begins talking to her; she misses the target and instead hits the teacher in charge of the class.
In the movie Bottle Rocket, there's a very touching scene outside of a bar between Anthony and Inez. The camera slowly pulls back, and through the massive window, we can see that inside the bar Anthony's friend Dignan getting completely owned in a bar fight after trying and failing to hustle some guys at pool.
The Steve Martin movie LA Story is full of hilarious background moments: During the opening monologue, Harris K. Telemacher is riding an exercise bike in an outdoor park (which has signs clearly stating "No Running"). While the camera is zoomed in on him, in the background a guy on a stationary rowing machine has a heart attack. An ambulance shows up, he's carted off, and another eager exerciser eagerly claims the now-vacant rowing machine. All in the space of 10 seconds, silently in the background.
After Walter pulls the gun on Smokey in The Big Lebowski, The Dude notes that the bowling alley has called the cops. During the conversation in their car outside afterward, the police can be seen through the back windshield coming into the alley.
An otherwise dour conversation scene in the (rather insular) Finnish movie Emmauksen Tiellä shows two unnamed men in the distance behind the actors, taking turns jumping on an anti-tank mine. The result is predictable.
In Gremlins, when Billy's dad in on the phone at his inventor's convention, in the background is a time machine (like the one used in the 1960 film, The Time Machine, also seen on The Big Bang Theory). The time machine disappears (having gone back in time) and people start feeling the air around where it was, looking for it.
KickAss: When Red Mist is trying to load the video from the "nanny-cam," take a look at the other videos saved on it...
In one scene in Love And Death, while one of Napoleon's advisers plots treachery, in the background, Napoleon is teaching his Body Double how to walk like him, and the two of them get in an argument that ends with a fistfight.
In the first Harry Potter movie, Harry and Draco are stuck wandering through the Forbidden Forest together and they stumble upon Quirrell drinking unicorn blood, who menacingly advances on Harry. The scene's rather terrifying... except for the part where you see Draco running across the background screaming like a pansy.
Even better when you consider that they're told Fang is a coward, and he's the one who brings help.
In the third film, as Harry stalks off furiously after blowing up his Aunt Marge, you can see Aunt Marge floating away in the distance. Also in the third film, during Buckbeak's first scene: most of the class backs off when Hagrid asks for a volunteer. Neville, still something of a Lovable Coward at this point in his character development, abruptly ducks out of sight behind a boulder.
The fourth film has something of a Triumphal Background Event that plays off the humor of a Funny Foreground Event: after Neville exclaims "I've killed Harry Potter!" while facing the area of the camera, Harry leaps out of the water in the distance, disappearing again before Neville turns around.
In the sixth film, Ron was under the effects of a Love Potion. Harry takes him to Slughorn, and while the latter two were talking to each other about Ron's condition, Ron, who was sitting on the sofa, falls on his back.
In the final film, after the battle is over and Harry, Ron and Hermione enter the Great Hall, Filch stands behind them, contemplating the enormous pile of rubble before him and then ineffectually sweeping the bit of floor at the bottom of it.
In the Bill Murray movie Larger than Life, one scene shows Bill Murray talking to the elephant while Matthew McConaughey is on the phone with his boss and he discovers that Bill Murray betrayed him. Watching McConaughey's exaggerated poses with the juxtaposition of the heartfelt speech from Murray is but one of the funny moments of this film.
In Fight Club, the Narrator is talking to Marla on the phone as Tyler dicks around with nun chucks in the back ground.
The unforgettable "Lovemaking Scenes" between Marla and Tyler, which are not on camera but HIGHLY audible when The Narrator is going about his daily life.
Scott Pilgrim vs. The World features several of these, two of the best being Scott diving through the window of his apartment to escape when Knives comes ringing (and then reaching back in to get his jacket), and the other being the Vegan Police doing a jumping high-five and shouting "YEAH!" when they leave after depowering Todd.
In the climax, one of the side characters remarks, "The comic is always better than the movie." In the pre-climax, the line was "Yeah, their first album was definitely better than their first album."
In Close Encounters of the Third Kind during the 'communication' scene near the end, when the mothership finally, and quite forcibly, answers the human communication attempts in kind, you see some random guy suddenly deciding that it would be safer to be in the Porta-Potty.
Jaques Tati's Playtime is entirely built on Funny Background Events. Tati wanted to make a comedy without a central character to focus on, so all of the humor comes from stuff in the background. And there are so many of them that you have to watch the movie over to catch all the visual humor.
In Teen Wolf, one of the extras in the basketball game scene exposed himself. This was part of the reason this movie was rented so much.
According to the IMDB's trivia page: "Immediately behind actors Michael J. Fox and James Hampton, an extra (in a red sweater) can be seen with an unzipped fly. This incident later resulted in an Internet rumor that the extra is a man exposing himself. However, the unmatted full frame version of the film reveals that the extra is actually a woman".
In Jekyll and Hyde... Together Again, Dr. Lanyon hooks his patient Mrs. Simpson to a device that would enlarge her breasts. Lanyon then leaves her unattended as Dr. Jekyll comes in to tell him about his transformation into Mr. Hyde, in fact transforming again while explaining the situation. Meanwhile...
In Idiocracy, when Joe goes to the hospital, the 'doctor' insults him for his low intelligence, then tells him that "There's lots of tards out there livin' really kick-ass lives. My first wife was tarded - she's a pilot now." Not ten minutes later, while Joe and Co are being chased by the police, a plane simply drops out of the air and crashes. Later in the movie, in Costco, the crashed plane is seen still stuck in Costco, where it crashed.
Even better, when in later viewings you realize the plane didn't just crash, but one of the cops fired their rocket launcher backwards and shot the plane down...
Bill and Ted's Excellent Adventure has sort of a Funny ''Foreground Event. In a scene where Napoleon Bonaparte is in the car next to Missy, he begins to sidle closer (likely intending to hit on her) while Bill and Ted make their way back to it. Napoleon promptly pulls back when they appear beside the passenger's side window.
The second Revenge of the Nerds movie combines this with an Overly-Long Gag. In one scene, the Alpha Betas are meeting at their local frat house, discussing their scheme to get rid of the Tri-Lambs, and during the whole meeting (which takes about five or so minutes), Ogre, the big, muscular member of the group, is in the bathroom behind them, peeing the entire time, only stopping at the end of the scene.
In Zombieland, when Tallahassee is smashing up the minivan the building behind him has several posters in the window related to the plague including one showing a grenade and the phrase "Solve It." It's even funnier when you see it's a bridal store.
In Who Framed Roger Rabbit, at the end, just after the wall into Toontown is broken, a train zooms across the field of view. If you pause and go frame by frame, you'll find a different crime being committed in each car (murder, etc).
In the very first scene of Spartacus, an extra can be seen slowly sliding backwards down the mountain behind the actors.
In the film version of You Can't Take It With You, as the family is packing up to move away, Rheba and Donald are having a conversation in the kitchen. Behind them, Kolenkhov is stuffing his pockets with sandwiches.
In The Waterboy, while Vicki is talking to a field reporter about her predictions on the outcome of the game, Bobby's mama walks behind them, notices she's on camera, and starts dancing to the marching band music.
In Super 8, while Charles is filming Alice and Martin at the train depot, Preston walks up to a pay-phone in the background and silently pretends to be speaking into it, but he's obviously just opening and closing his mouth.
In Woody Allen's Sleeper, while Woody's character is disguised as a robot butler, he attempts to deal with an out of control, sentient gelatin mold while Luna and one of her party guests have an obliviously-mundane conversation.
In Crank High Voltage, when an old lady is being interviewed about Chev Chelios' rampage, a goofy guy in the background smiles, waves, flaunts Eve's panties, and gives a thumb's-up.
A Monster in Paris: When the commissioner is holding a conversation with Pâté at Lucille's restaurant, a disgruntled waiter can be seen struggling to open a bottle of champagne in the background.
The Cabin in the Woods:Inverted. During the celebration, we continually see Dana getting brutally savaged in the background on the monitors, while the operators carry on partying in the foreground.
When the army of monsters is unleashed on the facility, quite a few darkly comedic moments can be glimpsed on the monitors: along with a Monster Clown giggling into one of the cameras, we see a monster with a jack-o-lantern for a head breathing fire at fleeing technicians, a huge creature stealthily creeping up behind an oblivious victim, the Sugarplum Fairy dancing balletically across a blood-splattered room, a couple of goblins taking one of the facility golf carts for a joyride, one of the Dolls hurling a molotov cocktail at a tied-down victim... and of course, Ronald the Intern holding up a sign reading "Help me! A dragonbat has my scent."
Reversed and Played for Laughs in Loaded Weapon 1, when in the background of one scene a woman is seen being asked by the cops to describe a suspect. She gives an outrageous description, claiming the suspect had big red lips, eyes as big as plates, and so on. In a later scene, a person looking like Mr. Potatohead actually having these features is arrested by the police. (And protests that he's innocent)
In the Alfred Hitchcock classic North by Northwest, during the scene in the Mount Rushmore cafe, a boy in the background puts his fingers in his ears right before one of the villains fires a gun... because Hitchcock shot eleven takes on that one scene, and the boy was apparently getting bothered by the noise.
In The Amazing Spider-Man, a delightfully aloof librarian carries out his business with noise-cancelling headphones playing classical music in his ear. He walks out of the Midtown High library, never realizing that Spider-Man and the Lizard have demolished the place mid-brawl, with Spidey even webbing a Lizard-flung desk away from the elderly man, very likely saving his life. This scene will likely go down as Stan Lee's best Creator Cameo ever.
Later in the movie, when Spidey is talking to Gwen on the phone, three girls in the background can be seen spotting him, pointing to him, and then taking a selfie.
In the beginning of Dumb and Dumber when Lloyd chauffeurs Mary to the airport, he turns his head and talks to Mary not to worry of anything while he runs a red light followed by an explosion behind the rear window.
When Harry and Lloyd run from the two criminals near the beginning a bum can be seen in the background pissing on a wall.
In the first Pirates of the Caribbean movie, at the climactic fight scene between the Pearl and the Interceptor, while Barbossa is looking all dramatic and shouting orders, a pirate can be seen behind him dangling in mid air from a bunch of ropes while his shipmates try to get him down.
When Murtogg and Mullroy begin arguing about how just because a ship has black sails doesn't make it the Black Pearl, we see Jack slowly slide out of frame, then appear in the background walking across the gangplank to the Interceptor and happily positioning himself behind the wheel. It's only then they realise he's no longer standing next to them.
In The Hobbit, Óin's Ear Trumpet was stomped on and crushed flat when the Company were captured by goblins. After when they escaped, Óin is seen using it, anyways.
Plenty moments in Bag End during the unexpected party. One involved Fíli and Kíli trying to figure out how to operate a tap on a beer barrel, apparently giving up and just bringing the entire barrel out.
When Bilbo suggests that the trolls skin the dwarves (to buy them time), Thorin is seen yelling and uselessly flailing in anger, along with the other dwarves (who are all on their backs except for Fíli who is desperately trying to turn over and failing).
In Jay and Silent Bob Strike Back Jay and Silent Bob attempt to leave the film shoot, as Ben Affleck and Matt Damon are filming Goodwill Hunting. A crew member is then seen escorting them back to their spot.
An odd, possibly unintentional one occurs in Hulk Hogan vehicle Mr. Nanny. During a short montage of Hogan's character riding his motorcycle, he passes by the ocean - behind him, a man can be seen throwing a dog in the water.
Even documentaries aren't immune. One of the Series/Up movies has the now-famous scene where one of the subjects' dog kills a rabbit in the bushes behind her during the interview.
During the finale of the Elvis Presley film Fun in Acapulco, Raoul can be seen dancing on a hat!
In Star Trek Into Darkness, when the experimental torpedoes are delivered to the Enterprise, Keenser is seen in the background sitting on one and looking rather taken with it while Scotty is pitching a fit at Kirk.
At the end of the Cold Open, the native casually dropping the sacred scroll as they start worshipping the Enterprise instead.
When Kirk and Scotty meet up with McCoy in sickbay after Spock detonates the torpedoes, Scotty glances down at McCoy's arm and goes wide-eyed. The doctor rearmed all the torpedoes the same way he armed the first one, getting his arm repeatedly caught by the sliding panels.
Subverted in The Hudsucker Proxy. Norville breaks the window in Mussburger's office, causing the pages of "the Bumstead Contract" to fly away. Trying to save them, Mussburger accidentally chases one out the window, in an apparent Ironic Echo of his boss's suicide earlier that day. Then it cuts to Bumstead himself downstairs, papers flapping past the window behind him, demanding to see Mussburger. "Or did he fall out a window too?" Instead of Mussburger falling past the window, we cut back upstairs and see that Norville has caught him by the legs. The scene then turns into an Overly-Long Gag as to whether he'll fall or not (he doesn't).
In Pacific Rim, as Gottlieb rattled off his predictions to Pentecost and Herc, Geiszler can be seen making "blah blah blah" motions with his hand. Likewise, in an earlier scene, when Geiszler gushes about how much he loves Kaiju to Raleigh, Gottlieb is shaking his head.
In Maleficent, when Maleficent and Diaval are about to storm King Stefan's castle, they begin arguing over who goes in (she wants him to stay out, he wants to go in with her), and Diaval lets go of the unconscious Prince Phillip to gesture with his hands. As he's speaking, you can see Phillip (who is magically enhanced to float like a balloon) start to drift away in the background.
Similarly to the page image, Hellboy II: The Golden Army has an early scene where the main characters are just strolling past BPRD agents wrestling with some kind of tentacle monster.
In X-Men, Magneto is giving out orders in his barren cave-office as a Newton's Cradle clicks back and forth on his desk. As he turns to leave, the little metal balls clatter to the floor, revealing that there were no wires and it was just him mentally fidgeting.
Combined with a Brick Joke in X-Men: First Class, Amy (the first woman that Charles tries to pick up at the beginning of the movie) shows up at his graduation ceremony and smiles at him—then Moira MacTaggert cuts in front to talk with Charles. Behind them, the blond woman leaves in a huff.