The best known example would be the War of Jenkins; Ear, where Britain went to war with Spain allegedly because the Spanish cut off the ear of Jenkins, who was the captain of a British ship. Jenkin then went to Parliament to complain about it and war was declared. (This war eventually merged with the War of Austrian Succession.)

What's important, though, is to differentiate between the cause of a war and the origins of a war. The War of Jenkins' Ear was not a war over an ear. The ear merely served as the immediate cause of the war, the cassus belli which would allow the two polities to fight about what they really wanted to fight about. The origins of the war go deeper, as at that time Spain's trade in its bits of the New World was exclusive. It was illegal for non-Spanish ships to ship Spanish New World goods, and even to be caught in Spanish New World waters (which happened to Jenkins, he was caught and accused of smuggling and so his ear got cut off). This trade was pretty wealthy, so plenty of non-Spaniards wanted in. The Jenkins' ear incident was just a convenience for the British, who didn't care about the ear except inasmuch as it was representative of their own trade ambitions in the region. They went to war for those ambitions, not for the ear. The eventual peace gave them leave to trade in the Spanish territories, but Jenkins himself was forgotten.

Moreover, there is apparently some doubt over whether or not the story of Jenkins ear actually happened.

I would imagine it was much the same between Bologna and Modena. Politics brought the two nations to war, the bucket merely provided the cassus belli.