Any era, any geographic location, any type of event [military, social, technological]. I'll take first whack:
December, 1241, Karakorum, Mongolia. Uggedai Qua Quan, third son of Temujin, Chingghis Quan, and Supreme Ruler of the Mongol Empire dies, most probably of the results of chronic alcoholism. Under the Jasagh, the Mongol legal code, all of the Princes of the blood, generals and Mongol nobility are required to return to the homeland for the convening of a Kuriltai and the election of his successor. Until that is done, his widow toregene Khatun acts as regent.
Word reaches the Mongol Army in Hungary in the Spring of 1242 of Uggedai's death. Among those who immediately de-camp are Guyuk, son of Uggedai, Kadaan, son of Chagatai [Temujin's oldest surviving son], and Mongke [oldest son of Tolui[Temujin's youngest son, and ruler of the Uluus comprising the traditional Mongol homeland. All take their personal armies with them. Faced with almost no Mongol troops to lead, Subodei calls off the campaign, and the Mongols withdraw to the east, razing Hungary, and keeping Russia. With one major exception, a raid, they never return.
The Mongols were on the cusp of taking Europe when they left. Never was so much of the Earth as likely to have been ruled as one polity, with Mongol overlords and Chinese administrators. And it was the death of one man, several thousand miles away, that stopped it happening.