I wasn't explicitly spoiled with The Sixth Sense but just knowing there's a twist I was able to figure it out before the end watching it where I don't know that I would've otherwise.
One recent one I thought was a let down was seeing the A-Team van destroyed in the trailer, it wasn't a major plot point but it ruined the scene.
Yeah knowing there was a twist in the Sixth Sense was enough for me to work it out the moment Bruce is shot, although in fairness to Shalyman he did enough to convince me I was wrong about half way through.
Luckily I never knew there was a twist in Fight Club, so it actually took me by surprise because I wasn't looking for it!
And what is it with giving away stuff in trailers? I love the pre title sequence to Casino Royale, but it's practically all shown in the trailer, which is a shame as the "The second is easier" bit would have been very cool as a surprise.