Bob plays Terror on Sue's War Mammoth, which would bury it. Bob doesn't want to respond to his own Terror, so Sue responds by playing Unsummon on it, which would return the Mammoth to its owner's hand (namely hers). Neither player wants to respond to Unsummon, so the batch is finished and starts resolving. First the Unsummon resolves and returns the Mammoth to Sue's hand. Then the Terror resolves, but it fizzles and doesn't do anything, as the Mammoth has left play.