Terminator: Salvation (2009):
The Story: In an effort to breakaway from prior Terminator films' present or near-future settings, Terminator: Salvation focused its story on an adult age John Conner (Christian Bale) who leads a strike against an experimental SkyNet facility - in the post-Judgment Day war where a new model of Terminator is being created: one with live human tissue covering its cybernetic endoskeleton. During the strike, Connor manages to free human test subjects that had been held prisoner by the machines - including Marcus Wright (Sam Worthington). A former death row inmate, Wright is given a second chance to help save humanity when he joins Connor and the other freedom fighters against the malevolent SkyNet threat.
The Spoiler: Marcus Wright is (unbeknownst to him) a Terminator.
The Trailer: The trailer for the film spends half of its runtime setting up the post-Judgement Day setting (as well as Connor's guerrilla rebellion against the machines) and the other half outright spoiling not only the film's most interesting twist (a Terminator that doesn't know he's a Terminator). Yet, the trailer doesn't stop there - since it also lays out Wright's entire character arc - specifically that he chooses his human friends over his machine creators.
The Island (2005):
The Story: Lincoln Six Echo (Ewan McGregor) and Jordan Two Delta (Scarlett Johansson) live in a futuristic compound, shielded from the outside world, which is no longer inhabitable for human life - except for an isolated island paradise where people still live in harmony with nature. Residents of the underground community follow a rigorous workout, sleep, and dietary regimen - in order to maintain top physical shape. Their incentive? Those who abide by the compound's overseers (a mix of scientists and military contractors) are eligible for entry in a weekly lottery - where one lucky resident is selected to leave the facility and join previous winners on the island. However, when Lincoln begins to suffer from mysterious visions of a different life, he starts to question everything he has been told about the island and the outside world.
The Spoiler: Lincoln Six Echo and Jordan Two Delta are clones.
The Trailer: While the trailer doesn't outright spoil why Lincoln and Jordan were created, the footage makes it crystal-clear that the pair are cloned "products" - and that the Island is a cover tale to disguise the compound's true purpose (extending the lifespan of wealthy customers by harvesting organs from living clones).
Free Willy (1993):
The Story: Following abandonment by his mother at a young age, Jesse (Jason James Richter) becomes a troublesome foster kid, living on the streets, bouncing from home to home, stealing, and vandalizing - even after being placed in the care loving parents, Annie (Jayne Atkinson) and Glen Greenwood (Michael Madsen). As part of his punishment for past crimes, Jesse is assigned to grounds maintenance at a local amusement park; though, when the troublemaker encounters the park's most notorious attraction, an unruly orca whale named Willy, the pair quickly form a close bond.
Jesse's care of the animal improves Willy's behavioral problems (and willingness to learn tricks) while the sea mammal helps his human friend find purpose and trust in his new family. Nevertheless, when the park's owner decides that Willy is more valuable dead than alive, Jesse and his family/friends must race to free the killer whale from captivity.
The Spoiler: Willy gets freed.
The Trailer: Given that Free Willy is a feel-good kids movie, it's no surprise that Jesse succeeds in freeing Willy; yet, trailers for the film left little suspense or doubt to the imagination - since the previews (and poster) place the climatic sequence front and center. Additionally, the Free Willy trailers don't just spoil how Jesse succeeds in liberating the titular whale, they also spell-out every up and down in the boy's transformation from a selfish and insecure kid to a sensitive and thoughtful caretaker.
Dawn of the Planet of the Apes (2014):
The Story: Dawn of the Planet of the Apes continues the story of Caesar (Andy Serkis) and his community of intellectually-enhanced apes. Following the simian flu plague, a byproduct of Dr. Will Rodman's experimental treatment for Alzheimers, humanity has stumbled back into a tribal society - without power, modern technology, or sustainable agriculture. In an effort to restore power to their San Francisco commune, Malcolm (Jason Clarke) and a group of simian flu-immune survivors start investigating the forest land outside of the city - until they begin to encroach on the apes' territory. As humanity and ape-kind converge, both societies are forced into a tentative truce - where one false move could mean all out war.
The Spoiler: Koba plays ape to play with guns.
The Trailer: While not an outright spoiler, Dawn of the Planet of the Apes trailers put heavy emphasis on several key sequences - that made it clear who would be responsible for violating the human/ape truce. The most egregious example of spoilery Apes marketing was the use of Koba (Toby Kebbel) placating human enemies with circus-like ape tricks - easily one of the best moments in Dawn of the Planet of the Apes.
Understandably, 20th Century Fox knew that such a clever movie moment would tempt casual viewers into theaters but there's no doubt that the overall impact of the scene (a significant turning point in the story and Koba's character arc) was undercut by pre-release previews that, across the board, played the sequence near-shot for shot.