In some unusual cases, the card you reveal from the top of your library may not have the same mana value as the spell your opponent cast, but you can still cast that card because the resulting spell does have the same mana value
Phantom Nishoba Loxodon Hierarch Nazahn, Revered Bladesmith Qasali Pridemage Saffi Eriksdotter Mystic Enforcer Knight of the Reliquary Fleetfoot Panther Gaddock Teeg Voice of Resurgence Rhys the Redeemed Tolsimir Wolfblood Wilt-Leaf Liege Sigarda, Host of Herons Sigarda, Heron's Grace Selvala, Explorer Returned Selesnya Guildmage Renegade Rallier Armada Wurm Captain Sisay Centaur Healer Dauntless Escort Dromoka, the Eternal Dragonlord Dromoka Enlisted Wurm Fleecemane Lion Karametra, God of Harvests Kitchen Finks Loxodon Smiter Mirri, Weatherlight Duelist Novablast Wurm Holy crap
Also great with your assorted Sagas
It also plays particularly well in a Wilderness Reclamation strategy
Then, with the other three, she casts Mind Rot, targeting you
