Possibility Storm issue: If the spell which triggers this ability is not on the stack when this ability resolves, the ability will properly reveal cards from the top of the Library... but it does not allow you to play a spell.
Example A: Two
Possibility Storm are on the Battlefield. Player casts an instant. Two separate instances of "possibility storm" triggered ability are placed on the stack. The first resolves properly... cards are revealed, then spell may be cast. Second ability resolves... cards are revealed, but you are not given the option to cast a spell for free.
Example B:
Possibility Storm is on the Battlefield, and I control a
Nivmagus Elemental. I cast an instant.
Possibility Storm triggers. I use
Nivmagus Elemental to exile the original spell and get 2 +1/+1 counters. When
Possibility Storm resolves, the cards are revealed, but you are not given the option to cast the spell for free.
Relevant
Oracle rulings from Wizards website: 4/15/2013: - "If
Possibility Storm's ability doesn't exile the original spell (perhaps because another
Possibility Storm already exiled it), you'll still exile cards from the top of your library until you exile a card that shares a card type with it and have the opportunity to cast that spell."