Highlander: Endgame, original trailer showed the villain, Jacob Kell, as some kind of sorcerer. Suffice it to say, they removed all of the “magic” in the released film. The trailer can still be seen with a quick YouTube search.
District 9.
The movie trailer I saw before another movie made it seem like a standard issue shitty action movie. It was not.
The trailer for Speed made it look like garbage when it was actually really fun.
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Amazing no one has mentioned Bridge to Terabithia (2007). The screenwriters disavowed the trailers, iirc. Looks like a cute kids fantasy movie, starts out leaning that way with a middle-school Manic Pixie Dream Girl, then turns hard into being completely real life and extremely tragic. 14-year old me was not prepared.
Mother! (2017) with Jennifer Lawrence was not even the same genre I thought it would be.