WB's trailer that spoiled Doomsday in Batman V Superman.
Can you imagine the surprise, the sudden ratcheting of tension and scope, when the movie you thought was a mutant combo of Dark Knight Returns and World's Finest turns out to secretly contain Death of Superman?
The movie didn't service Doomsday or the Death of Superman well, as an arc. Doomsday's only strength in the film would have been a surprise reveal. That second trailer spoiled that and burned the surprise for the sake of hype.
Then, WB studio management had enough lack of self-awareness to turn around and bitch about how much they spent on marketing, spoiling the one word-of-mouth surprise hook of the movie. Plus, they developed expectations that his brief appearance as a final macguffin couldn't support.
In fact, it was his appearance in the trailer that was the final clue the movie was, as Jeremy Irons put it, "a little overstuffed."