The ripping of the peace sign poster uncovers a small surprise. It may be considered a goof because Gavras’ film tells the story of Lambrakis’ assassination in 1963 and Leone’s film released three years later, but Z was officially “inspired” by the actual events and took place in a unnamed city with unnamed main characters in unspecified year – so technically not a goof.
Sergio Leone’s The Good, the Bad and the Ugly (1966) in Costa-Gavras’ Z (1969).