The Gods Must Be Crazy is a collection of epic fails every ten minutes of the movie. Ay ay ay ay ay. What's worse, roughly half the wacky predicaments ensue simply because at the beginning of the movie, someone throws an empty Coca-Cola bottle from an airplane. Seriously. We never even learn the litterbug's name.
In the Garfield movie, there's a scene near the middle of the film where Garfield is upset over Odie. He proceeds to vent this frustration by smacking Odie's favorite ball. The ball knocks an object over, then that object knock something else over, and this chain reaction continues until the entire wall-high shelf tips over and onto Garfield. When Jon comes back inside, he is not happy. Garfield lives, of course.
Iron Man 2 gives us the Ex-Wife, a tiny yet massively explosive missile... that gets shot at Vanko but does absolutely nothing. Not even explode. The disgust in Tony's and Rhodey's voices is palpable.
In some of the tie-in comics that led up to Iron Man 3, It actually shows off a working Ex-Wife. Apparently, that particular one was a dud. The real Ex-Wife in action...well...
In the Fantastic Four movies, Ben mentions that Johnny "washed out of NASA for sneaking two Victoria's Secret wannabes into a flight simulator. They crashed it into a wall. A flight simulator."
Mulholland Drive has a scene where a hired killer kills his victim and then tries to fake suicide by putting his gun into the victim's hand. While trying to do so, he accidentally pulls the trigger and the bullet goes through the (fairly thin) wall into the neighboring flat, hitting a fat, ugly woman. He then goes into said flat and attacks the woman, who turns out to be stronger than he expected. He eventually overpowers her and tries to drag her into the flat where he killed the first guy in order to fake a suicide-with-murder scenario. While passing the floor, he is observed by a janitor who apparently doesn't speak English, but slowly follows him into the flat. Back there, the killer first kills the woman, then the janitor enters the room with his vacuum cleaner. He also shoots the janitor, but by accident, the vacuum cleaner is turned back on again. He then pulls the janitor into the room. Finally, he tries to turn off the vacuum cleaner... by shooting at it. The vacuum cleaner catches fire, which sets off the fire alarm.
In Snatch... pretty much anythingSol and Vinnie put their minds to. In their defining moment, they were hired to take a briefcase from a man who had been sent to place a bet at a bookies. They get the cash from the bookies, their employer gets the case. Unfortunately, their getaway driver, Tyrone, Drives Like Crazy and backed into the van their victim had used to get to the scene, knocking him out and trapping him inside it. Later that night they see someone carrying a case go inside, and without attempting to verify the target, they enter the bookies, only to find that it's (obviously) the wrong person, and furthermore that the bookies has no cash because all bets are off. Then the cashier lady turns out to be a Badass Bystander who deftly disarms Sol of his shotgun and trips the alarm. Then they attempt (and fail) to open the front door. Reasoning that it's a security door that locked when the alarm was pushed, they try to Shoot Out the Lock. The door turns out to be bulletproof, and Vince gets his leg grazed by the ricochet. They fall down in exhaustion and take off their ski masks, at which point they notice the security camera that's just caught them both unmasked. And then, to top it off, Tyrone shows up to get them... it turns out the reason they couldn't open the front door is because they tried to push the 'pull' side of the door. The pincher is that the pair are completely unknown in that part of the underworld, and the camera fails to be of any use because the owner doesn't recognize them... but Tyrone is recognized. Epic Fail indeed.
Pretty much every single plan Jason conceives in Mystery Team. Most notable is when a speech on how a certain character won't shoot them gets him shot
In the 2008 Get Smart movie, Maxwell Smart is given a tiny grappling hook launcher, and while using it to free himself, manages to hit everything except what he was aiming for, including a secret button that drops him out of an airliner. In flight.
Made slightly more epic by the fact that the grappling hook launcher was part of a Swiss Army Knife. You know, the thing with a blade on it! For cutting!
Monty Python and the Holy Grail: The knights are crossing the bridge of death and must answer the bridge keeper's three questions, and if they fail any one they will automatically fall off the bridge to their death. The first two questions always ask one's name and quest, but the third is always random, ranging from something impossibly easy like "what is your favorite color" to a ni-difficult "what is the capital of Assyria." Galahad lucks out and manages to get the "favorite color" question... and still manages to screw up his answer, leading to him plummeting to his death. About as justified as this trope can get, since he was just fluidly and unthinkingly repeating the previous knight's answers to the same questions and didn't realise that wouldn't work for favorite color until it was too late.
From Pulp Fiction, one of the guys at the apartment where Jules and Vincent go to retrieve Marsellus Wallace's briefcase ambushes them and shoots at the duo at very close range... and misses every shot. All three pause to stare in surprise.
In Repossessed Ernest and Fanny Weller, two corrupt televangelists (parodies of Jim and Tammy Faye Bakker) decide to televise a real exorcism and take phone donations. All is going well until Fanny demands a chance to sing on television. The money donated swiftly starts to go down.
The entire US military does this in the American movie version of Godzilla. First they miss the skyscraper sized dinosaur with rockets and heat seeking missiles from close range and destroy more of the city than the lizard does. Somehow, the monster was able to hide in a city of eight million.
In Star Trek III: The Search for Spock, Kirk and his True Companions steal the Enterprise. The shiny new Excelsior, in hot pursuit, powers up its revolutionary transwarp drive... which promptly conks out due to Scotty sabotaging it beforehand, complete with sputtering engine noises, leaving Starfleet's "Great Experiment" adrift and having to be towed back to Spacedock.
The Lord of the Rings: The Two Towers: When the Ents (gigantic living trees) hold a council, they demonstrate just how mind-bogglingly slow they are at doing anything.
Treebeard: We just finished saying..."Good morning."
The 1989 film: Batman programs the computer in his Batwing to fire two machine guns and several missiles at The Joker. Every missile and bullet completely misses. Even more humiliating when the Joker causes the Batwing to crash to the street below with a single shot from his (admittedly very long) gun.
Batman Returns: The Penguin snatches up an instrument panel from the wreckage of the Batboat and tries to use it against Batman, assuming it's some kind of weapon. It turns out to actually be the device that overrides the frequency jam on the Penguin's radio signal to his pet penguins to launch their missiles at Gotham City, and as a result the penguins fire their missiles at Penguin's own hideout, completely destroying it.
In X-Men: First Class, after Erik has just stopped a missile strike from the US Navy, Xavier tries to get Erik to call off his counterattack by saying that "they were Just Following Orders!" He said that to a Holocaust survivor. Magneto's face and quiet voice show everyone just how utterly Xavier has failed to convince him, before he returns the missiles to sender.
In The Man with the Golden Gun, Hip and his nieces rescue James Bond from Hai Fat's dojo. The plan goes well, until Hip drives off without Bond, despite his nieces trying to tell him that the man they were sent to rescue has been left behind.
Toy Story 2: "Prepare to meet, Mr. Angry Eyes!" quoth Mr. Potato Head as he rushes to the attack in Al's apartment. After switching his regular eyes... for an extra pair of shoes. So naturally he just runs into the furniture and looks like an idiot. Jessie's WTF facial expression makes it even better.
In The Men Who Stare at Goats, Lyn crashes into a rock in the middle of the desert, the only such object for miles around. It was probably destiny.
Trying to break Simba and Nala up, Timon from The Lion King 1½ throws a beehive at them...only for the hive to go ahead and the bees to stay right above his head. Why this happened...the world may never know.