On the morning of April 14, 1561, residents of Nuremberg, Germany reported that they saw an aerial battle occurring in the skies. According to them, there were hundreds of spherical and cylindrical objects moving erratically in the sky. In addition, there was a large crash outside the city.
A few interpretations of this event include light refraction through clouds of ice crystals, or swarms of insects. Overall, it's a very enigmatic event, and has been one of many interesting UFO sightings in history.
Sources:
Dennett, Preston (2008). UFOs and Aliens. Infobase Publishing Company. p. 20.ISBN 978-0791093849.
Vallee, Jacques and Aubeck, Chris (2010).Wonders in the Sky: Unexplained Aerial Objects from Antiquity to Modern Times. Tarcher. ISBN 1585428205.