In 1963, six years after the second Exclusion Act, the Pacificon II Committee announced that they were banning Walter Breen from the convention. The committee announced their intention in a PR well before the Worldcon was held, explaining that they had been advised that they might be held liable if Breen (who was known by many to have had sexual relations with boys) were to seduce an underage male fan there, but also plunging all of active fandom into war.
At around the same time, Breen was blackballed by the 13 members of FAPA needed to drop him from FAPA's waiting list, but within a very short period of time a different group of 13 blackballed the entire waitlist. The Secretary Treasurer took notice of the fact that FAPA had no waitlist, and came up with a handy list of names — which "just happened" to be the original waitlist (before Breen was blackballed) in the original order. Breen took on a de facto membership anyway when he married Marion Zimmer Bradley, who was already a member.
Despite protests and even outright boycotts by some, Breen was not allowed to attend the Pacificon II. Bill Donaho outlined the committee's actions, detailing incidents which had been observed regarding Breen that fell short of seducing youths but nonetheless concerned the committee, in a pre-convention fanzine called The Boondoggle (full title: The Great Breen Boondoggle or All Berkeley Is Plunged into War). This was marked as a letter-substitute and also marked as DNQ: nevertheless, the resulting fandom-wide War is often referred to as the Breendoggle or the Breen Boondoggle.
Although Breen's behavior at conventions right around the time of Pacificon II seems to have been beyond reproach, Breen (who also wrote an authoritative book on man-boy love) was known by many fans, especially in the Bay Area, to have engaged in sex with boys. (Ultimately, he died in prison a multiply-convicted pederast.)
But even 40 years after the event, the sole point fans on both sides can agree upon is that the resulting feud had long-lasting effects, including leading to a proliferation of mutually exclusive private apas where the opposing forces retired to lick their wounds and assure themselves that they had been undeniably right while the other side had been unmistakably wrong. Even Donaho later (by Feb 10, 1964, per a letter to Alva Rogers) came to believe that the expulsion was "both ethically wrong and stupid."
But over the years, more evidence has become public and we can now be reasonably sure that he was guilty of substantially what he was accused of. (Perhaps most damning, Marion Zimmer Bradley, one of his vigorous defenders at the time, later testified under oath that she was aware of his activities.)
By today's standards, of course, the Pacificon II committee took the minimum possible action. The controversy over the expulsion seems to have had a number of sources, but the main one seems to have been that, while Breen's behavior was known to some, it was not known to all fans or even to a majority. Only ten years after Joe McCarthy, fannish tolerance for unsubstantiated accusations was very low, so when the first that many people heard about it was an official committee publication, the reaction was decidedly mixed.
See Sam Moskowitz' Pacificon II reminiscence for some additional information on this.
See also Exclusion Acts