What happens when you wear armor you’re not proficient in?

What penalties or restrictions does a creature have when they don armor they’re non proficient in?

For example, picture a newly adventurous (level 1) Dwarf Barbarian (18 Strength, 10 Dexterity) who has inherited a set of Full Plate armor. He’s only Trained up to Medium Armor, but he feels obligated to don it. What are the results? What if he still wears this armor at level 10 (assuming he never takes the Armor Proficiency Feat)?

This is generally a question to clarify related rules, but it has the possibility to come up. For instance, said Barbarian may invest in Feats that give benefits when hit or critically hit. Additionally, you could ‘equip’ the enemy Wizard you’ve captured to add to your restraints.

Can you wear a Mithral Chain over Snakeskin Tunic and have their effect stack?

Can you wear a Mithral Chain over Snakeskin Tunic and have their effect stack?

Snakeskin Tunic: A snakeskin tunic is a tight, form-fitting shirt crafted from the scales of a giant snake. When worn, it grants a +1 armor bonus to AC, a +2 enhancement bonus to Dexterity, and a +2 resistance bonus on saving throws against poison.

Mithral Shirt: This extremely light chain shirt is made of very fine mithral links. The armor has an arcane spell failure chance of 10%, a maximum Dexterity bonus of +6, and no armor check penalty. It is considered light armor.

Can a character wear both Chain mail and Leather to get a higher AC and to fool an enemy?

Background:
I am reading the anime book "Goblin Slayer", the light novel not the comic.

In it the main character wears leather armor over plate or chain to both stack his AC and to confuse his (not very intelligent) enemies into hitting where leather is vulnerable but plate or chain is not.

Question:
In dnd 5th edition, can one wear multiple types of armor and get an AC stack? Can it confuse weak foes like goblins?

Notes:
Here are answers that don’t speak to this question:

  • Can a chain shirt be concealed under normal clothing? (this is about concealed, I’m asking about stacked)
  • Stacking multiple types of armor (this is not dnd 5e centric)
  • Can you wear a Mithral Chain over Snakeskin Tunic and have their effect stack? (this is pathfinder)

Please, if you are going to down-vote, say why in the comments. If I don’t know what I did wrong, it is literally impossible for me to willfully act better and change that thing. It might happen by accident, but the universe doesn’t like happy accidents.

What in-universe reasons exist that explain why druid can use metal weapons but not wear metal armour?

In 5e, we are told that the druid shouldn’t wear metal armour:

Armor: Light armor, medium armor, shields (druids will not wear armor or use shields made of metal)

I believe this was true in older editions of D&D as well; "druids will not wear armor or use shields made of metal". Certainly the below quote seems to imply this…

In the 2016 sage advice, we are told that druids choose not to wear metal armour:

What happens if a druid wears metal armor? The druid explodes.

Well, not actually. Druids have a taboo against wearing metal armor and wielding a metal shield. The taboo has been part of the class’s story since the class first appeared in Eldritch Wizardry (1976) and the original Player’s Handbook (1978). The idea is that druids prefer to be protected by animal skins, wood, and other natural materials that aren’t the worked metal that is associated with civilization. Druids don’t lack the ability to wear metal armor. They choose not to wear it. This choice is part of their identity as a mystical order. Think of it in these terms: a vegetarian can eat meat, but the vegetarian chooses not to.

This question is not about what happens if druids wear metal armour, or whether certain druids might choose to wear it despite not being a common choice among druids.

My question is: why is there not a similar taboo around druids making use of metal weapons? Is there anything in any published material (ideally from 5e but I suspect that previous editions probably have more to say about this than 5e) that explains why druids are generally happy to metal weapons, despite the fact that they typically choose not to wear metal armour (or use metal shields)?


Just a reminder that this is not a designer-reasons question as I’m interested in lore-based answers, in-universe explanations, not any designer’s reasons from any edition as to why it was decided from a gameplay-based or mechanical point of view. I’m interested in the lore reasons only.


Related:

  • What would be the side effects on a Druid of wearing metal armor?
  • If I multiclass from ranger into Druid, can I still wear metal armor?

[ Politics ] Open Question : I don’t wear a mask, I play football, and I go out with some women and I didn’t get the coronavirus. Does this mean that it is a invention?

I don’t know anyone who had a corona and I’m not going to use these ridiculous masks. My health is that of an ox. Shouldn’t I have died according to the socialist media?

Can a Centaur wear and benefit from magical boots, slippers and other non-horseshoes?

It seems agreed upon that Centaur player characters (from Guildmasters’ Guide to Ravnica & Mythic Odysseys of Theros) can wear, and benefit from, magical horseshoes (of speed / of a zephyr).

But what about other types of footwear : slippers of Spider Climbing, boots of speed, etc ? Could a Centaur player character benefit from those too ?