In The Poverty of Historicism, while talking about the uncertainty of progress, Karl Popper wrote "For we cannot exclude the logical possibility, say, of a bacterium or virus that spreads a wish for Nirvana" (1957, p 157). What is interesting is that Popper was infected with this virus—initially, Marxism, gradually decreasing in virility to socialism and then interventionism.
In 1914, at the age of 12, Popper read Edward Bellamy's utopian novel, Looking Backward:2000-1887 (Hacohen, 2000, p 67-68); and, without guidance, it is hard to believe that he would have been able to engage it critically. In fact, in his intellectual autobiography, he criticizes himself for his uncritical acceptance of Marxism (1982, 33-34). In that same book he demonstrates that while into his seventies he has still not rid himself of the disease, writing "... if there could be such a thing as socialism combined with personal liberty, I would be a socialist still" (36).
As far as I know, Popper, while sometimes mentioning free markets positively, never condemned socialism for its negative economic consequences that have been resulted each time it has been tried. Even the voluntary socialist experiments of the 19th century (e.g. by adherents of Owenism in the U.S.) and of Zionists in Israel have been, if not outright failures, difficult to sustain. Perhaps his rejection of economism—the idea that economic welfare is a guide for social policy and a characteristic he identified in Marx's writing— led him to believe that such a criticism was unnecessary and would reduce him to Marx's level.
The failure of Popper to embrace the economics of Mises and Hayek has led to the perpetuation of error, most visibly in the person of George Soros, and put humanity at risk of another dark age. Edward Bellamy would be astonished.
Hacohen, M. H. (2000). Karl Popper — The Formative Years, 1902-1945. Cambridge: Cambridge University Press.
Popper, K. R. (1957). The Poverty of Historicism. Boston: Beacon Press.
Popper, K. R. ( 1982). Unended Quest. La Salle: Open Court.