A similar thing happened to me recently, actually. I kept seeing the previews for that Shooter movie and thinking "oh cool, a new Matt Damon movie that isn't The Bourne Whatever, I'll probably see that." When it came out on dvd I ended up renting it with a friend of mine and we were both incredibly disappointed and surprised to find out that it was fucking Marky Mark Wahlberg. I spent like a year thinking it was Matt Damon, despite the posters in the cinema and the video store not having any mention of his name.