The story of the Fourth Crusade is interesting. The Crusaders had contracted with Venice for transport and general logistical support for 85,000 silver marks. The Venetians, under the  85-year-old and blind Doge Dandolo, wanted the money up front. However, the crusaders could only come up with 49,000. Dandolo made a deal. A city in what is now Croatia, Zara (now Zadar) had revolted against Venetian rule, and would the crusader army take it back. Despite the crusader oath promising not to attack other Christians, some of the crusaders did exactly that and sacked the city. When Pope Innocent III heard of it, he excommunicated everyone (and then lifted the excommunications a couple months later.)