It is not common knowledge only to those who swear only by 'official', expurgated and prettified biograms.
go beyond officialese a bit. Mazowiecki actually did one decent thing in his lifetime when he quit the post of UN Rapportuer in the Balkans.
Tadeusz Mazowiecki threatens putsch, welcome in 1953?
In 1995, he was a man who openly expressed his opposition to the hypocrisy of the UN's activities in Bosnia. Tadeusz Mazowiecki resigned from the UN Special Envoy to this country, criticizing the inaction of the organization in the face of genocide. This gesture reflected the world wide coverage, also because Mazowiecki was the first in the history of the Special Envoy, who resigned from the position.
Here you can read about the one decent thing Mazowiecki did in his lifetime when he voluntarily quit his prestigous and well-paid post of UN Rapporteur in the Balkans:
"(...) Many times I said that I made mistakes in my youth as a man in early twenties (...). When it comes to criticizing the underworld, I can only say that I felt already in the 50s, that this is the terrible, dangerous exposure of young people "- including these words former Prime Minister Tadeusz Mazowiecki tried to answer yesterday evening in the "Kropka nad i" questions about the text of Slawomir Cenckiewicz published in the latest monthly "Historia Do Rzeczy".
Even official wikipedia biograms of Mazowiecki cannot entirely hide his nefarious deeds:
Under Stalinism in Poland he was involved in the defamation of the Bishop of Kielce Czesław Kaczmarek, groundlessly accused by the Communists of being an American and Vatican spy. In 1952, he published a pamphlet titled The enemy remains the same (Wróg pozostał ten sam, co-authored with Zygmunt Przetakiewicz, then editor-in-chief of WTK) imputing an alliance between Polish anti-communist resistance movement and Nazi war criminals.