Old English texts (c.700-1100 AD) frequently describe sword blades as poisonous, and Megan Cavell has published a great discussion of these sources. She ponders whether these texts are describing a real practice -- we know, for example, that sword blades were etched with corrosive acid to bring out the patterns in their metal, so might this acid be the poison described? Or perhaps the texts refer to an archaeologically invisible practice of actually poisoning blades before battle.
Personally, I don't think these texts refer to actual poisoned blades. Instead, I think they describe the (morally) poisoning effects that violence had on weapons. As swords shed human blood, they became tainted -- poisoned -- in the eyes of the Christian monks who described them. Monastic authors were very cautious about weapons -- they affirmed their importance, but also warned (in Solomon and Saturn II, for example) that weapons could sumon the devil. In this text, the author explains that drawing a sword would release the devil unless the sword's bearer recited the Lord's Prayer as he pulled the weapon from its sheath. The language in this poem -- describing the rules etched into swords blades -- echoes that used to describe poisoned swords in other texts like Beowulf. To these authors, the liquids that iron came into contact with could corrupt the metal -- blood get into a sword, and changed its physical properties. If that blood were human, improperly shed, the sword could become tainted. Hence blades were poisonous because they had come into contact with and absorbed something from the human blood that they shed, not by literal poisons applied to their edges.
But perhaps there was a more literal reality behind this metaphor. We can't know for sure -- swords from archaeological contexts are too corroded for any trace of a poison to survive. Perhaps Cavell's musings are correct and swords were sometimes actually, literally, poisoned to increase the harm they could cause.
It was not uncommon during the early days of muzzle loaded muskets/aquebuses/rifles for soldiers to keep their lead bullets inside rotte meat/corpses for a period of time so that even a scratch could generate a kill on the long run because of infection. This was particularly the case during the 30 years war. This started to be seen as dishonourable already by the Napoleonic wars and there were previous conferences opposing this practise. However this did not stop individuals from occasionally doing so and sometimes bullets would be infected involuntarily by being exposed to infected water (waterloo saw a few of these cases).
Source: Tim Clayton's "Waterloo"
Magellan was allegedly shot with poisoned arrows and stabbed with poisoned spears when he died on Mactan Island in 1521. However we only have Antonio Pigafetta's account of the battle. In any case, even if the arrows and spears were not poisoned, we at least know that the practice was either known, or thought to be known at the time.
http://www.history.com/this-day-in-history/magellan-killed-in-the-philippines