FIRST APPARITION [an armed head]
Macbeth! Macbeth! Macbeth! Beware Macduff,
Beware the Thane of Fife. Dismiss me. Enough. (4.1.1)
You'd think that Macbeth would have learned to be wary of weird (in both ways) visions and creepy, bodiless ghosts. Instead, he just takes what they say and runs with it. It seems like he's got a pretty bad case of confirmation bias.