Arcane Bombardment

If you cast a spell with {X} in its cost without paying its mana cost, you must choose 0 for X.

The spell that caused Arcane Bombardment's ability to trigger is normally still on the stack while that ability resolves. That spell won't be among the cards in your graveyard when you exile an instant or sorcery from it at random.

If you've already cast an instant or sorcery spell the turn that Arcane Bombardment enters the battlefield, it won't trigger until you cast an instant or sorcery spell on a later turn.