You stab someone wearing a breastplate at the neck, shoulder, or armpit, the only reason those are not considered a joint is a person wearing just a breastplate isn't wearing enough armor to have joints.
a) an immensely lucky arrow shot
b) another knight or man at arms physically throwing him to the ground, grappling around for a bit, then stabbing that fuck in the eye.
Those two examples have enough repeated occurances in history that entire graveyards can be filled for each example seperately!
And there are also cases that don't fit, there are armored corpses with lance-tips, axe blades, knives, and other weapons embedded in the visor found buried in ancient battlefields, and there are records of peasants drafted into an army besting knights, the weapons which they used was a footman's spear, a small hand axe, and a dagger(required to be shorter than a sword)
Armored corpses with lance tips and axe blades in their visor plate make perfect sense. Axes are mass weapons with wedge shaped heads, against armor they were basically like flanged maces except slightly less effective. To get the most out of your axe, you would need an overhand swing, or if you were mounted you could afford a side swing. Now as for lance tips in visors - consider that the primary use of the lance was on the massed cavalry charge, typically directed at infantry formations. The head is a prime target for a man-at-arms on a horse charging a man-at-arms on foot.
As for records of peasants being drafted into an army besting knights - don't make too much of it. In the first place, the 'peasant army' is mostly a Victorian era myth, perpetuated by terrible writers like TH White, or a product of the "dark ages", where limited power bases meant limited pools from which to draw one's soldiers. In that earlier period, 'knights' as you know them didn't really exist, nor did most helmets have 'visors'. The most technically complex heavy armor available was chain mail, unlike the ubiquitous plate armor that arose during the later middle ages.
Armies were composed of professional soldiers - mercenaries - or wealthy commoners with tools intended to kill - the footman's spear you mention is just as likely to have been a bill or some other kind of pole arm, specifically meant to defeat armor and cavalry, and they were armored, while more lightly, just like other the men-at-arms.
As an aside, I'll address this point:
You stab someone wearing a breastplate at the neck, shoulder, or armpit, the only reason those are not considered a joint is a person wearing just a breastplate isn't wearing enough armor to have joints.
Someone wearing a breastplate is probably also wearing a gorget or an aventail, as well as spaulders or pauldrons. Sure, you could stab someone in the armpit - but they also wear underarmor, padding and mail, specifically to cover up joints. This is why I mentioned halfswording: to successfully kill someone with an armpit stab, yoou either need tremendous luck or you need to grasp your sword by the blade and
shove it through the other guy's underarmor.