I've heard recently about the relative weightlessness and flexibility of full plate armor, in comparison to chain mail, due to weight distribution and the use of separate armor pieces. I have also heard that after the initial charge, many knights would remove their helmets because of the restriction they put on visibility.
However, I was always taught that during the Battle of Agincourt, many French knights were killed because they were knocked face down in the mud, and were unable to get up due to the weight of their armor in the mud. The muddy water seeped into the points in their helmets, through the air holes, and they were drowned.
These two "facts" seem somewhat contradictory to me, and I would simply like some clarification on the issue.
Edit: I use too many commas
The impact of the mud at Agincourt was that it slowed down the French advance. Plate armor isn't "weightless;" the general estimates I've seen for late medieval plate range from around 50-60 pounds. That's not an insignificant weight, but neither is it too heavy to prevent a man from getting on a horse or swinging a sword with relative ease. However, in muddy terrain like at Agincourt, the additional weight would be more of a burden. The mud slowed the French infantry down, which allowed the English archers to get in more volleys. Every new bunch of arrows coming in caused further casualties, disrupted the unit more, and generally lowered morale. By the time the French slogged up towards the English men-at-arms (men whose lines were in fairly good order and who were not tired from the advance or wounded by missile fire), they were severely disadvantaged. Even with that, enough of the French made it to the enemy to make a rather brutal fight. At that point, the mud gave the English one more advantage: their lightly armored archers could drop their bows, take up swords, mallets, and axes to fight as light infantry. The archers could move much more rapidly through the mud than the men-at-arms, which allowed them to reinforce gaps in the line or outflank the enemy. I don't think that the image of French knights literally drowning in the mud is entirely mythological, but also I don't think that drowning would be one of the main causes of French casualties in the battle. Anyone getting knocked to the ground by a pole-axe or mallet too close to the English lines would be in more danger of being taken prisoner or outright executed where he lay than drowning.