The “accidental” poster reveal was via an Instagram Live post by Tom Holland, who plays Spider-Man in Infinity War, in which he receives the poster from Mark Ruffalo, reveals it to the camera, and then appears to realize it wasn’t supposed to be released.
Shortly after the “accidental” reveal was posted, Marvel officially released the poster.
Marvel also announced today that the first official trailer for the film would be released on Wednesday. The announcement was made with a trailer for the trailer collecting fan reactions to the trailers for previous Marvel films.