Both of these require you to know what you’re looking for, which might be cleverness.
The Bird With The Crystal Plumage-
The killers hands are rendered perfectly in line with the reveal near the end of the killer’s identity.
Meet The Robinsons-
The matriarch of the Robinson family is shown on the poster in her young form, rather than the old form which is comtemporary to all the other family members.
Not a poster but the DVD menu for The Boondock Saints would spoil something for you if you had never seen it before.
The voiceover bit they chose was “Police are looking for three men the media have dubbed “The Saints.”” The picture that goes along with this is Connor, Murphy and an older guy with a big beard and scary sunglasses. The older guy you are supposed to think is a villain when you first see him. The reveal that he is actually on the side of the brothers (because he’s their father) comes at almost the very end of the movie. I guess they’re counting on people to not scrutinize that DVD menu too closely.
Since they already know that the watchers are people who do not scrutinize their DVD movie selections too closely, they probably think it is a good bet. 
The worst is the poster for The Hudsucker Proxy. It shows the mysterious invention that Barnes is working on for most of the film.
Not a poster, but the trailer for the recent Total Recall remake gives away the plot twist that Colin Farrell’s wife is not really his wife. Of course, if you had seen the original film, you’d know that anyway, but not everyone has seen the original. Revealing that info in the trailer seemed an odd choice to me. But then again, after having seen the movie, I realize that there are precious few good lines in the new Total Recall, so they were probably desperate for trailer fodder.
Slightly off-topic, but people who aren’t familiar with them might enjoy seeing a group of posters that tend to give away absolutely nothing about the content of the movie: Polish film posters.
Related to that is the DVD menus for 24. They would always have a scene or two repeating in the background of the menu, and it would often spoil you to what happens. Like someone important getting shot, or kidnapped, or what have you.