A tanú (The Witness) is a Hungarian cult film directed by Péter Bacsó, and one of the funniest political satires ever made. It was created in 1969, in a tense political climate at a time when talking about the 1950s and the 1956 Revolution was still taboo. Although it was financed and allowed to be made by the communist authorities, it was subsequently banned from release. Rumour spread about the existence of the film, and it became popular even before being released, its popularity probably contributing largely to its release ten years after it was made. It was screened at the 1981 Cannes Film Festival.
Here is the New York Times review from 1981.