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The Great Breen Boondoggle was an incident circa 1963 in which fan Walter Breen (soon after, the husband of very well-known fantasy writer Marion Zimmer Bradley/MZB) was banned from Pacificon II/Worldcon as he was judged to pose a threat to children. The action was led by Bill Donaho, the chairman of Worldcon, and The Great Breen Boondoggle is the title of the zine that Donaho wrote about the action.
The exclusion was highly controversial and Donaho reportedly regretted it. Fancylopedia 3 writes (at 15 June 2014):
Donaho later (by Feb 10, 1964, per a letter to Alva Rogers) came to believe that the expulsion was "both ethically wrong and stupid."… By today's standards, of course, the Pacificon committee took the minimum possible action.
Breen was indeed a child abuser: at the time of Boondoggle, Breen had a little-known 1954 conviction for child molestation. He was arrested again in 1989, pleaded guilty and placed on probation, arrested again in 1990 and convicted. He died in prison in 1993.
The text of The Great Breen Boondoggle is available at the Breendoggle Wiki.