Like, you've seen bits of certain anime (pics/gifs) or heard a thing or two about it, and thought - "yeah, this anime gotta be about this and nothing else", but then you actually see it and it turns out its way different than you thought.

Some of mine would be:
JoJo Bizarred Adventure - Before I watched it, I was thinking that this anime is about angry mexicans who beat each other to the death and on top of that, search diamonds in desert with Indiana Jones (on steroids) help and talking dog...
Neon Genesis Evangelion - for some reason, I thought that this anime was about giant spaceships fighting in galaxy...
Monogatari series - I was thinking that this is mystery slice of life about group of high school friends who are being killed/targeted by some monster, which was one of them... And the so called monster actually was
Rezero - this I guess was my stupidity, because when I saw cover for the first time I thought "oh, another harem anime". After couple of day I came back and this time read its synopsis, and thought "wow, this just might be a good a good harem" (I still havent checked out genres at the time...). Well, after this I finally tried to watch it and... Yeah, a big surprise was waiting for me.