The biggest example is probably Tari Tari. It had this very generic and terrible trailer, but the show itself was good.
I guess you could criticize them for not making their trailers have enough interest. I think a lot of anime trailers really don't show the true potential of the show. It would be doubtful one would watch Clannad just from trailers, for example. Many more examples.
I know, right? I'm people have been thrown off by Hentai Ouji's title and the premise suggested by its trailer, and there are people who think that it's wise to look at a teaser to judge a light novel series (and it's blatantly wrong).
Lesson learned: Choose wisely and hold on to your intuition. Being pleased or disappointed relies solely on the watcher itself.
I think we've leaned to never judge a show by its trailer! Don't people know most shows tend to have multiple PVs and the first is almost always a rough sketch that introduces the title and characters?
Hmm... I think I can see this as a challenge.
Since I'm the kind of person who doesn't drop a series as long as possible, I'll try watching Mahouka to see if I can actually make it through without dropping it.