It is perhaps natural for a modern student of medieval combat to assume that a thrown spear would be relatively ineffective against 15th century plate harness. However, when I put the question to a practical test with a reproduction spear, I discovered, somewhat to my surprise, that I consistently got better penetration against sheet steel with a thrown spear than with a two-handed thrust with the same weapon. Two factors seem to explain the difference. With a thrust, the spear must begin to decelerate before the arm reaches full extension if the wielder is to keep a grip on it. Probably more significantly, the thrown spear can be thrown as the thrower runs forward, for a substantial increase in velocity and kinetic energy.
An Olympic level javelin throw carries about 360 joules of kinetic energy. According to tests by Alan Williams, even half that is sufficient to drive a point 40 mm through 2 mm of mild steel. This would be sufficient to threaten the limbs of many 15th c. harnesses, and Monstrelet gives an account of this happening in a combat between John de Merlé and the lord de Chargny in 1435:
Everything being ready, the king-at-arms, called Golden Fleece, proclaimed in three different parts of the lists that all who had not been otherwise ordered to guard them should quit the lists, and that no one should give any hindrance to the two champions under pain of being punished by the duke of Burgundy with death. Eight armed gentlemen were appointed to take or raise up either of the champions, as the judge of the field should direct. When the proclamation was made, the lord de Chargny issued out of his pavilion with both of his two weapons, holding his axe in his right hand, the iron part toward his adversary, and thus advanced a little forward. The Spanish knight advanced at the same time from his pavilion, armed as aforesaid, having a kerchief thrown over his helmet that covered his visor, which was half raised. And as he came out one of his servants removed the kerchief. They made for each other with vigorous strides, brandishing their lances; but the Spaniard all this time had his visor raised. And as they came together the lord de Chargny threw his lance first without hitting his man. The Spaniard advanced to throw his. He hit him on the bracer near the elbow: with that stroke he was pierced and a little grieved in the arm, so that the lance was lodged within the bracer. But the lord de Chargny shook it off and it fell on the sand. The two champions now vigorously approached near to each other: that is how they nobly began to fight with their lances. But the lord de Chargny was much displeased that his adversary did not close his visor.
From: Monstrelet, Enguerrand de, (La) Chronique d'Enguerran de Monstrelet. Paris 1857 Translation copyright 2007 Will Mclean
A longer but less accurate translation of the encounter is available here.
Also, Olivier de la Marche records how the Lord de Habourdin, bastard of St. Pol, did arms at Bruges against the Bernard de Bearne, bastard of Foix in a continuation of the pas de la Pelerine of 1449:
.. the bastard of Bearne came out of his pavilion with his visor closed, his lance gripped in his right hand, and an axe and small steel targe (targon) in his left, and he was a tall and powerful knight. On the other side the bastard of St. Pol came out completely armed, the coat of arms of Luxembourg on his back, a basinet on his head, without a visor or other cover or protection for his face, and he was armed with lance and axe, and equipped with a small steel targe. The two knights went fiercely against each other, and the Lord de Habourdin threw his lance first. And Sir Bernard stepped aside so that he was not hit, and as he did so he brandished his lance and threw it at his companion who was quickly following up his throw with his small targe in front of him as cover. And it happened with that throw the Lord de Habourdin was struck on the lower edge of his targe, and the blow glanced off and struck him on the left side a little below his waist. It pierced his harness and the point entered deep into the flesh of de Habourdin. When this happened he coolly used his left arm to pull out the bloody lance which was firmly stuck in his harness.
Oliver de la Marche, Memories Paris 1884
Translation copyright 2007 Will Mclean