Looking for a shot that I didn't solve and didn't bookmarked, I visited the profile page of the poster (I remembered who it was). The shot was among his/her "favourite movies" because it was the only shot available for this movie. And I got the solution :)
To help the poor programmers in avoiding the half horse head, I give this additional information: this happens only from the profile page: the 3 thumbnails displayed under the title "favourite movies" link directly to the movie page. On the opposite, if you display all the favourite movies, then the thumbnails link to the shot page and you cannot have the title before 30 days.