What happens:
You can no longer cast spells/skills granted by items
How to reproduce:
1) Have an item equipped granting you a skill in that combat tree
2) Get a magical item that grant you +1 in a combat skill and equip it
3) learn that combat skill via a book since you now have the required stats due to the granted +1 from the magical item
4) deequip +1 item, this causes your skill to unable to be used via the item
specific example:
-have migo's ring equipped so you can cast restoration.
-get +1 hydro from whereever (i got mine from talisman)
-now with 1 in hydro, u can learn restoration via skillbook
-after learning the spell and removing your boots. It is now impossible for you to cast restoration via migo's ring since the spell is greyed out.
When you deequip migo's ring afterwards it says that "some skill requirements are no valid, skills have been ummemorized"....But restoration wasn't memorized to begin with, it was part of the skills granted by items.