Answer: The Viking invasions
Explanation: While the Vikings were important and fascinating, the extent of their influence on shaping medieval European history is very overstated. They are often portrayed as having a more profound impact than they might have had in the broader context of medieval developments. This perception can overshadow the significant contributions of other cultures and empires during the same period, making the Viking invasions a candidate for one of the most overrated phenomena from 600 CE to 1200 CE.