Trailers have spoilers (both big and small), and/or are outright deceitful about the movie.
(Regarding deceitful, you might've seen amateur trailer cuts that, say, make a light comedy look like a dark thriller, but the professionals were doing that first. How would a director have cut this, if they were making a more marketable picture than was actually made?)
Similarly, I avoid seeing any reviews until either after I've watched the movie, or after I've started and am ready to abort it. I want to experience the storytelling, and also form my own impression, before someone spoils either for me.
I do often vet a title first by looking at its ratings pair on https://www.rottentomatoes.com/ (RT), and occasionally I look at the one-sentence review summary blurb. Especially for streaming services in recent years, where the majority of the titles are either mediocre or poor.
(SPOILER ALERT: Though, vetting with RT won't always save you from a bad experience. The other night's evening wind-down light movie, I picked what looked like a generic Jason Statham film, IIRC without vetting. And around halfway in, I was horrified, when the formulaic gruff antihero's redemption-lite arc suddenly reversed, to a double-down of violent and unnecessary pure evil, upon some innocent child. Then I went to skim the RT's summaries of professional reviews, and even none of those summaries warned of this.)