One of the wounds on Richard III's skeleton is a shallow cut to to his right mandible. I believe that this was very probably inflicted when someone cut away his helmet strap with a double-edged dagger, leaving him helmetless. Seven other wounds are still visible on his skull and most if not all were inflicted after he lost his helmet. At least two would have instantly put him out of the fight, and been fatal soon after.
The nearly contemporary romance Tirant lo Blanc also notes this vulnerability of the sallet style helmet popular at the time. In the narrative, one of the protagonist's combats is recounted to a hermit: both the hero and his opponent tried to cut the other's helmet cord. The tactic was common enough that even the hermit, who has never borne arms but has passed some time with a skilled knight, knows a remedy: the helmet cord should be made of flexible wire wound with silk cord like a ribbon.