Fałszywka (English: false document or forgery) is a Polish socio-political term describing counterfeit top secret files and fake police reports produced by the Communist secret service in the People's Republic of Poland. Their purpose was to undermine the popularity of prominent opponents of the ruling Party, mainly by attempting to ruin their good name as private individuals. Fałszywka (pl. fałszywki) used to contain seemingly stolen or declassified revelations about opposition members working as alleged police informants under the Soviet system. Most notably, an entire forged file of this sort was produced in the 1980s and then disseminated by the communist establishment about the leading dissident and future President of Poland Lech Wałęsa when he was nominated for the Nobel Peace Prize. This file was fabricated and then "leaked" to the media (as "proof" of his betrayal of Solidarity) in an attempt to prevent Wałęsa from being awarded the Prize. A similar file led to the publication of a widely successful book Zabić tego Polaka (To Kill that Pole) about the fake assassination plan against the Pope John Paul II during his 1983 pilgrimage to Poland.
More information and some examples on the Wikipedia page  - with some discussion on the talk page.