Responding to the First Witch, regarding when they (witches) will meet again, the Second Witch replies, "When the hurlyburly's done; / When the battle's lost and won." (I.i.3-4) This line could refer to the fact that Macbeth has won the battle and his enemy has lost. But it could also refer to the fact that although Macbeth has won, this victory is a curse in disguise because it will lead to subsequent tragedies. So, even if he's won, he loses in the end.
At the end of the scene, all three witches chant:
Fair is foul, and foul is fair.
Hover through the fog and filthy
This is the most famously quoted paradox: really two paradoxes - "fair is foul, and foul is fair." It simply means that what seems to be a good omen or a good event ("fair") is actually going to be bad ("foul"). And the reverse will also be true: what seems foul is actually fair. Macbeth will receive prophecies from the witches about his future rise to become king. This seems very good but as it is paved with tragedy and deception, it is quite foul.