Which event from Acts II and III demonstrates the recurring motif of Fair is foul fair?

In Act II, Scene ii, when Macbeth first encounters the witches, they hail him as "Thane of Cawdor" and "king hereafter." This prophecy sets in motion the chain of events that leads to Macbeth's downfall. However, the witches also warn Macbeth that he must "beware Macduff." This warning foreshadows Macbeth's eventual defeat at the hands of Macduff.

In Act III, Scene iv, Macbeth meets the witches again. This time, they show him a series of apparitions that predict his future. The first apparition is a head that warns Macbeth that he will not be defeated "until Great Birnam wood to high Dunsinane hill Shall come against him." The second apparition is a bloody child who tells Macbeth that he will not be defeated by "any man of woman born." The third apparition is a crowned child who holds a branch from Birnam Wood. This apparition tells Macbeth that he will be defeated by "Macduff, born of woman."

These apparitions play on Macbeth's fears and insecurities. They convince him that he is invincible, which leads to his downfall. The motif of Fair is foul fair is evident in these scenes because the witches' prophecies initially appear to be positive, but they ultimately lead to Macbeth's destruction.