What: If you try to "use item" on a claim you will get the message "You cannot cast from stored items" as would make sense. It does however, use the charge of the item in question.
When: Recently noticed this on a varnish, but then confirmed it was also possible with a consumable item that was looted then stored later, (specifically a glow jar in this case).
Where: seems to be location independent.
Recreate: Store an item that has charges (though I've so far only tried with single use items) then "use item" with the claim. You get the error message you can't cast it, but it will still expend the charge.
