The witches predict that Macbeth should be wary of Macduff, as he is a potential threat. This comes true when Macduff leads an army to overthrow Macbeth.
They also predict that Macbeth will not be defeated until Birnam Wood comes to Dunsinane. This comes true when Malcolm's army uses tree branches from Birnam Wood as camouflage to approach Macbeth's castle.
