The slip-up comes after Glory dies, but before the portal is opened. After Doc (the demon who cuts Dawn) throws Spike off the tower, he moves to Dawns left side (the viewers right). When Buffy reaches the top of the tower, he turns to her. He is on her right side (the viewers left) and is holding a knife in his right hand. He then lunges to the LEFT, allowing Buffy easy access to throw him off the tower. Since he was standing on her left, and he lunged to the left, he had no actual way of stabbing her.