In the Central Library of Zürich there is a broadsheet print by Hans Glaser depicting a strange and fantastical looking mass of geometric figures flying in the sky above a town. The text below explains how the print is not fantastical, but depicting as accurately as possible, the events described by a large enough number of witnesses that they cannot easily be discounted.
Since the events are described with precision by Glaser himself, it's best to quote the broadshit rather than to talk about it.
"In the morning of April 14, 1561, at daybreak, between 4 and 5 a.m., a dreadful apparition occurred on the sun, and then this was seen in Nuremberg in the city, before the gates and in the country – by many men and women. At first there appeared in the middle of the sun two blood-red semi-circular arcs, just like the moon in its last quarter. And in the sun, above and below and on both sides, the color was blood, there stood a round ball of partly dull, partly black ferrous color. Likewise there stood on both sides and as a torus about the sun such blood-red ones and other balls in large number, about three in a line and four in a square, also some alone. In between these globes there were visible a few blood-red crosses, between which there were blood-red strips, becoming thicker to the rear and in the front malleable like the rods of reed-grass, which were intermingled, among them two big rods, one on the right, the other to the left, and within the small and big rods there were three, also four and more globes. These all started to fight among themselves, so that the globes, which were first in the sun, flew out to the ones standing on both sides, thereafter, the globes standing outside the sun, in the small and large rods, flew into the sun. Besides the globes flew back and forth among themselves and fought vehemently with each other for over an hour. And when the conflict in and again out of the sun was most intense, they became fatigued to such an extent that they all, as said above, fell from the sun down upon the earth 'as if they all burned' and they then wasted away on the earth with immense smoke. After all this there was something like a black spear, very long and thick, sighted; the shaft pointed to the east, the point pointed west. Whatever such signs mean, God alone knows. Although we have seen, shortly one after another, many kinds of signs on the heaven, which are sent to us by the almighty God, to bring us to repentance, we still are, unfortunately, so ungrateful that we despise such high signs and miracles of God. Or we speak of them with ridicule and discard them to the wind, in order that God may send us a frightening punishment on account of our ungratefulness. After all, the God-fearing will by no means discard these signs, but will take it to heart as a warning of their merciful Father in heaven, will mend their lives and faithfully beg God, that He may avert His wrath, including the well-deserved punishment, on us, so that we may temporarily here and perpetually there, live as his children. For it, may God grant us his help, Amen. By Hanns Glaser, letter-painter of Nurnberg."
From Colman S. Von Kevicsky, "The Ufo Sighting Over Nuremberg in 1561" Official Ufo, January 1976, pp. 36–38, 68. The translation is by Ilse Von Jacobi.
The phenomenon has been interpreted in various manners throughout the centuries. As an admittedly biased ufologist, it might be easy to jump to the conclusion that this is the description of a battle between alien aircrafts, however it is never good to focus on one possibility and ignore all others. First of all, we must acknowledge that we don't have a lot of sources for this event. Even the Glaser print isn't a first-hand account, and it is fairly unspecific in telling its own sources, too. Since we are talking about a relatively distant past, that is to be expected, but it does mean we must take this account with a grain of salt.
With that said, to simply excuse and handwave the phenomenon away as an example of mass psychosis seems too easy, as well. It is possible that this was some sort of biological phenomenon that the inhabitants of Nuremberg didn't understand.
As a matter of fact, only five years later, something similar happened in Basel, in Switzerland. In 1556, people in Basel saw unusual sunrises and sunsets and described black circles "fighting" in the sky. And in general, many leaflets have been found describing similar events in Europ in the 16th century. At the time, these events were described and perceived to be religious in nature. Some people today have excused them as possibly being of military origin, but it seems natural to me that the people at the time would have known what a cannonball was, and would not have found these phenomenons so strange if they appeared to just be normal battles happening in the distance.
While there is not enough substance to these descriptions to say anything, it is worth noting that black triangles have been described even in modern times by a few people who witnessed UFOs. The nature of them has never been fully understood.