The first apparition conjured by the witches in Act 4, scene 1, tells Macbeth to "Beware Macduff."  The second apparition says, "Be bloody, bold, and resolute. Laugh to scorn / The power of man, for none of woman born / Shall harm Macbeth."  Therefore, the first spirit warns Macbeth about the Thane of Fife while the second tells him to go ahead and feel confident because no man born of woman will be able to harm him: an apparent contradiction.

This is a trick.  It is true that Macduff will be the one to kill Macbeth; however, the second apparition's statement is so enigmatically worded that it sounds as though Macbeth has nothing to fear because, of course, every man was born of a woman.  However, Macbeth doesn't realize that Macduff was born via Caesarean section, and so he wasn't -- in an extremely literal and technical sense -- "born."  The witches hope to make Macbeth feel secure so that he does not put up his guard.  If he feels safe, then he will actually be more vulnerable and liable to make mistakes.