Cardinal Baldassare Cossa was one of the seven cardinalss who, in May of 1408, deserted Pope Gregory XII, and, with those belonging to the obedience of Pope Benedict XIII, convened the Council of Pisa, of which Cossa became the leader.
Edward Gibbon asserts in The History of the Decline and Fall of the Roman Empire that John XXIII was charged with piracy, murder, rape, sodomy, and incest... with the more serious charges being suppressed. Of course, this needs to be viewed in the light of the political situation of the time, as the charges were likely trumped up; note the similarity to the charges against the Knights Templar.
He should not be confused with Pope John XXIII of the 20th century.