IN 1869 there was a question about who would succeed Isabella II of Spain. One candidate was a Leopold Prince of Hohenzollern. He was apparently initially uninterested in the prospect, but Bismark saw commercial and military advantages for Prussia should Leopold take the throne. At Bismarcks urging king William I ordered Leopold to accept the Spanish offer, all done apparently without French knowledge.
Apparently Bismarck initially planned the offer and acceptance to be all done before the French could react without seriously offending the Sovereignty of the Spanish people, leaving them to gripe but make it a fait accompli.
However, here is where the small event had huge consequences. It was a process that took a while to negotiate. Prim, the leader of the ministers of Spains national assembly the Cortes, had them in prolonged session that summer, until the news of Leopolds acceptance made it possible to proceed to formal election. On the 21st of June 1870, the Spanishes main supporter of Leopolds election Salazar, telegraphed Bismarck that things were proceeding well, but that he would be back in Madrid (where it was all to be legitimized), by the 26th.
Supposedly, by an error of deciphering this last date was read as the 9th! I don't know how you can easily mistake the 26th for the 9th, or if there were more conspiratorial possibilities.
The couldn't realistically hold the assembly over for another two weeks doing nothing, so it was disbanded until the autumn. As it happened, the news was leaked just a few days after Salazar returned, and Pim had to inform the french of their offer and Leopolds acceptance for damage control.
It became a huge controversy in France and Germany, but war could still have been avoided. Leopold actually backed down, but the French demanded 'satisfaction', a guaranty of some sort from King William that no such candidature should be considered again, at Ems on the 13th of July. With a big of creative editing of a telegram (and sending it to a bunch of newspapers!), both parties went away feeling insulted, war fervor was whipped up in two nations, and two days later, the french voted for war credits and it was on.
Some seem to claim, the whole candidature was a ploy by Bismarck to goad the french to war, but this gives him too much credit, though he did take advantage of the situation when it went sour. The French could have embarrassed Bismarck badly, and might have gained allies if they hadn't been so haughty. As it was, the French nearly voted against the war credits, but by then they knew how Bismarck had edited the telegram and that he wanted war.
Some may argue that a conflict between France and Prussia was inevitable (as both seemed to want war), but does that make it so? They were certainly rivals and France was seen as risking becoming a second rate power, while Prussias star was ascendent. Is war between Russia and America inevitable? Those may be interesting historical questions.
So in summary, a typo decyphering led to a major European war leading up to WW1.