Very little full armour was as all-covering as Henry VIII's "spacesuit" armour. Many armours (especially for cavalry) left the backs/insides of the thighs and the buttocks unprotected. Other common gaps were insides of elbows, knees, armpits, groin. Since the wearer needs to see out, there will be a gap (even if small) for vision.
Armour is not completely impenetrable. Torso armour tended to be arrow-proof and helmets were often arrow-proof, but arms and legs were often more lightly armoured. Even where armour was arrow-proof, there were weapons that could defeat it.
So, some ways to die in battle while wearing full armour:
Be crushed by your horse when it falls with you in the saddle.
Be crushed by an elephant.
Fall off a scaling ladder or castle wall.
Drown.
Die of smoke inhalation in a burning building.
Take an unlucky arrow through your leg armour into a major artery.
Be wrestled to the ground and stabbed by a dagger through your eyeslot.
Take a sword or spear point in your unprotected (or lightly protected) armpit.
Die of heat exhaustion.
Be crushed by the crowd.
Be shot by a musket/arquebus.
Be shot by a cannon: https://collections.royalarmouries.org/object/rac-object-13744.html
Be hit by a large rock thrown by a catapult.
Have a large rock dropped on you from a castle wall.
Take a hard hit from a polearm - bills and halberds and various spiked polearms can and will go through armour. (There are helmets out there with square holes, possibly from warhammers or other spiked polearms. Could also be crossbow bolts.)