Macduff was not born of woman - he was delivered by Caesarean section.
The witches tell Macbeth that no man born of woman can harm him. Sounds pretty good - all men are born of women, right? All men, all people, came from a woman.
However, the term "born" literally refers to the process of going through the birth canal. Macbeth believes he is safe, until he meets Macduff, who the witches told him to watch out for. Macduff proves that it is possible for man not to be born of woman:
Macduff: Tell thee, Macduff was from his mother's womb Untimely ripp'd
In other words, he was pulled directly from his mother's stomach, not "birthed" or "born".
This is the final example of how Macbeth has misinterpreted events, twisting what he has heard to suit his own purpose - but in the end, his misperception has sealed his downfall.
In Act 4, scene 1, Macbeth returns to the witches to consult them about his future. It is at a time when he has become paranoid about his security and wishes to hear their assurances. The witches call up different apparitions and the second apparition informs him:
"Be bloody, bold, and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth."
This statement emboldens Macbeth and he says:
"Then live, Macduff: what need I fear of thee?"
It is ironic that Macbeth refers specifically to Macduff here, since he discovers to his utter dismay later, that Macduff was not "of woman born".
When the two confront each other on the battlefield in Act 5, scene 8, Macbeth warns Macduff:
"Let fall thy blade on vulnerable crests;
I bear a charmed life, which must not yield,
To one of woman born."
"Despair thy charm;
And let the angel whom thou still hast served
Tell thee, Macduff was from his mother's womb
Macduff informs Macbeth that he had not been naturally born, i.e. he did not pass through the birth canal but was prematurely removed (ripped) from his mother's womb - so it could not be said that he was 'born' in the true sense of the word. It was an unusual and unnatural act. It could be that Macduff's mother was incapable of bearing her son by natural means which compelled the surgeons of the time to perform, what we call nowadays, a Caesarean Section.
Macbeth is shocked and dismayed by this information and realizes that he had been fooled by the witches. They had deliberately misled him - an example of the equivocal and paradoxical nature of their predictions. He declares:
"Accursed be that tongue that tells me so,
For it hath cow'd my better part of man!
And be these juggling fiends no more believed,
That palter with us in a double sense;
That keep the word of promise to our ear,
And break it to our hope. I'll not fight with thee."
When Macbeth refuses to fight, Macduff commands him to surrender but Macbeth prefers not to. They fight and Macbeth is killed.