1509:

Henry VIII marries Catherine of Aragon

After his father's death, and before he was officially crowned king, Henry took the widow of his brother Arthur to wife. The first marriage may never have been consummated, but Henry would later use it in his arguments for an annulment. The pope would not grant him his annulment, and his divorce from Catherine would result in a breach that spawned the English Reformation.

This event was suggested by John Herrman. Thanks, John!