In Mordenkainen's Magnificent Emporium, there are a bunch of new armor and armor types. It says that to wear these armors, you need to take special feats.
What happens if a PC finds this armor as a reward, but has not yet taken the feat? Is there just a penalty to AC, or are they also unable to use the special property? (like barbed)
Also, what happens if a level 5 character stumbles upon a level 10 magic item, or finds a store that sells a level 10 magic item and has the gold to purchase it. Does the magic item not work? Are they not allowed to buy it?