A bloody episode in a long war introduces the idea that defenses designed to protect against one kind of hazard can render their users prey to other kinds of danger, usually not foreseen by those who created them or even appreciated by those who use them. In short, defenses can be dangerous. This is no less true now—in the age of high-technology systems—than in 1415.

On a damp late October morning in 1415, a considerable force of heavily armored French cavalry advanced towards a small and sickly English army made up of 5,000 lightly clad archers and around 300 men-at-arms (the French army was at least five times this number). The mounted French knights rode across ground bounded on either side by thick woods. Although they had intended to attack the flanks of the English army to avoid the well understood threat of their longbows, the terrain caused the two wings of the attack to bunch together in the center of the field. When they came within range, the English archers loosed a storm of yard-long steel-tipped arrows. Some of the French were killed outright, but many were thrown from their disabled horses.

By the early fifteenth century, the plate armor worn by knights had almost reached its zenith of weight and sophistication. It was proof against most penetrating and edged weapons but had a fatal flaw. The armor was so heavy that its unhorsed occupant found it difficult to get to his feet, particularly in the confined and muddy conditions that prevailed on the battlefield of Agincourt. Once on the ground, they lay helpless and were slaughtered by the unencumbered English foot soldiers armed with mallets, spikes, and daggers.
(Some were taken prisoner and then killed.) While the English army lost around 100 men and boys in the battle, the French dead, mostly nobles, ran into many thousands.

Source: MANAGING THE RISKS OF ORGANIZATIONAL ACCIDENTS, James Reason, 1997