My favourite scenario from "Missives from Possible Futures #1: Alternate History Search Results by John Scalzi" is definitely #6.
Scenario #6
Event: ADOLF HITLER is KILLED by BULLET WOUND IN CROSSFIRE BETWEEN TIME-TRAVELING ANTI-NAZIS SENT BACK TO KILL HIM AND TIME-TRAVELING NAZIS SENT BACK TO PREVENT HIS ASSASSINATION
As a result: Causality loop annihilates time and space surrounding Vienna, knocking everyone in the city back to 1529 and the eve of the First Turkish Seige; as the 20th century Viennese use their historical knowledge to help the 16th Century Viennese, time-traveling pro-Viennese forces appear and fight a pitched battle with time-traveling pro-Ottoman forces, pushing everyone back to 955 and the Battle of Lechfeld; when the time-traveling pro-Magyar forces show up, they are slaughtered by everyone else which is tired of all this time-traveling crap, thereby ending the causality loop. Vienna becomes world power; Henry Jasomirgott first man on the moon, 1155
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