For obvious reasons, all examples contain spoilers, so they will be unmarked.
The 2004 Platinum edition of Aladdin starts with several movie trailers before you reach the main DVD menu, including the trailer for The Return of Jafar, where he, you know, returns. As a genie.
And Heaven forbid anyone watch Aladdin: The Series before seeing that Iago did a Heel–Face Turn in The Return of Jafar. The Disney Channel aired several episodes in April 1994 before Disney's video department released that sequel in May, and thus Iago was inexplicably being friends with the crew, perching harmlessly on Jasmine's shoulder, etc. Even the cover for that film doesn't hide that fact.
The plot twist where Syndrome kidnaps Jack-Jack overlaps with the Jack-Jack Attack short, and it seems everyone is perfectly willing to discuss it openly.
Several people called the Kari phone calls and subsequent Jack-Jack and Syndrome scene a bonus materials deleted scene for the DVD on first viewing of the film. Lo and behold...
The trailers for Incredibles 2 show that Jack-Jack has superpowers, something that was supposed to be an unexpected surprise in the first film.
On the 2012 Diamond edition of Lady and the Tramp (both on the DVD and Blu-ray discs), it's best to start the movie immediately if you don't want to know that Lady and Tramp have kids, Tramp gets taken in by the Darlings for his effort in protecting the baby and that Trusty survives after getting hit by the dogcatcher's wagon, as the main menu shows a clip of the final scene in the movie, which spoils all three. The former spoiler even comes less than a minute into it!
The Land Before Time: Want to know if the heroes made it to the Great Valley in the end of the first film? Don't watch any of the 13 sequels then. Or the TV series.
Trailers and merchandise for the movies after the first one both obviously and inevitably spoil what was a huge surprise in the first movie: Fiona turns into an ogre. "Why is Fiona fat and green?! Wait! NOOOO!"
Merchandise released before Shrek the Third included a toy set of Shrek and Fiona's children, the Ogre Triplets, who don't appear until the VERY end of the movie.
The official Wreck-It Ralph coloring book goes through the entire story, spoiling everything. The Wii, DS and 3DS tie-in games came out a week before the film. Since they were set post-movie, most of the major twists were spoiled, mainly the true identities of Vanellope and King Candy.
The true identity of Vanellope is referenced a few times in the sequel, which may be confusing to viewers who haven't seen the original film. There's even an entire scene devoted to the Disney Princesses thinking that Vanellope is a princess, which was shown in many of the trailers.
The closing credits of Winnie the Pooh (2011) credit Huell Howser as the Backson, spoiling The Stinger in which it is revealed that he isn't just a figment of the 100 Acre Wood gang's imagination.
Monsters University, a prequel to Monsters, Inc. made several years after the original, revolves around Mike enrolling in college in order to become a scarer. If you've already seen Monsters, Inc., you know Mike never gets to be a Scarer. On the other hand, if you watch Monsters University before watching Monsters, Inc., you will have one of the latter movie's plot twists spoiled for you, namely Roz actually being a CDA agent.
In their October mailings, the Disney Movie Club advertises their movies by the villain. This can spoil movies where who the villain is is a plot twist, including Toy Story 3, Up and Frozen, as well as the true name of Bowler Hat Guy.
Tangled: In the commercials during the Latin American TV premiere, the music video of the song "You're The Glow" was shown, which contained shots of sun rays coming from Flynn's body and a short-haired Rapunzel kissing Flynn.
If you had never seen Frozen before watching Season 4 of Once Upon a Time, then it wasn't long before the plot twist that Hans is the villain was spoiled. Frozen II similarly never tries to hide said plot twist either.
Disney Fairies had Vidia reform by the end of Great Fairy Rescue and make her a member of the core group, unofficial second in command at that.
The "Women of Inside Out" video in the Inside Out Blu-ray spoils Joy allowing Sadness to touch the core memories at the end.
LEGO clearly didn't mind spoiling the 2003 BIONICLE movie Mask of Light, as the promotional stills seen in the building manuals of several 2004 toys are taken from the end of the film, when Takua puts on the mask and becomes the Seventh Toa he had spent the movie searching for.
In order to establish the canonicity of the movie, Season 8 of My Little Pony: Friendship Is Magic spoils several of its events and the fate of one of the characters within the first few lines.
A mural within the Pixar Pier at Disney California Adventure depicts a scene that gives away the ending of Coco. It depicts Miguel standing proudly with the (both living and deceased) members of his family, dressed in full mariachi garb and guitar in hand, suggesting that the Rivera clan's music ban will be lifted at the movie's end. It also shows Mama Coco as a spirit, spoiling her eventual passing, while the inclusion of Hector among Miguel's deceased relatives risks spoiling the twist of him being Miguel's great-great-grandfather — who gets spared a Second Death by the time the movie ends, and instead finally acquires permission to revisit the Land of the Living on Dia de los Muertos. In turn, Ernesto de la Cruz is not on the mural, potentially spoiling the fact that he is not Miguel's great-great-great-grandfather, as Miguel starts off the movie believing.
Most merchandise from the first movie, as well as all media in the franchise released after it, shows Branch with his original blue hair and skin, which he turns to in the actual movie when he sings to save the lives of the other Pop Trolls.
Likewise, plenty of promotional material and merchandise for Trolls World Tour and following media don't hesitate to show Barb's Heel–Face Turn at the end of the movie.
The plot of The LEGO Movie 2: The Second Part relies a lot on the context of Finn and his sister playing with the LEGO toys, while the entire live action world was a climactic reveal in the first movie. Since the sequel picks up right where the first movie left off, it also spoils the surprise of the Sequel Hook. This movie also consistently calls Wyldstyle by Lucy.
Brave: The trailers and official website go to great pains to hide a number of details about the plot, but the toyline and at least a handful of pins (including one for Father's Day) are quite clear about the fact that Elinor and the boys are turned into bears.
