For such a short scene, act 1, scene 4 of Shakespeare's Macbeth abounds with irony. At the beginning of the scene, Duncan makes a remark to his sons and others about the traitorous Thane of Cawdor:
DUNCAN. There's no art
To find the mind's construction in the face:
He was a gentleman on whom I built
An absolute trust. (1.4.13-16)
Macbeth enters at that moment.
The audience knows that Duncan has given the Thane of Cawdor's title and possessions to "worthy Macbeth," and the audience also knows that the new Thane of Cawdor is no less treacherous and traitorous than his predecessor.
A little later in the scene, Duncan announces that he has selected his eldest son, Malcolm, as heir to his throne:
DUNCAN. ...Sons, kinsmen, thanes,
And you whose places are the nearest, know
We will establish our estate upon
Our eldest, Malcolm, whom we name hereafter
The Prince of Cumberland; which honor must
Not unaccompanied invest him only,
But signs of nobleness, like stars, shall shine
On all deservers. (1.4.41-48)
Shakespeare uses Duncan's passing reference to "stars" to turn Duncan's turn-of-phrase against him:
MACBETH. ...That is a step
On which I must fall down, or else o'erleap,
For in my way it lies. Stars, hide your fires;
Let not light see my black and deep desires... (1.4.55-58)
Later in the play, Shakespeare revisits one of Macbeth's words: "o'erleap."
The word appears only once more in the play, in Macbeth's "If it were done when 'tis done, then 'twere well/It were done quickly" soliloquy at the beginning of act 1, scene 7, in which Macbeth explores a number of reasons for not murdering Duncan.
At the end of the soliloquy, Macbeth indicts himself for having no other reason for murdering Duncan and taking his throne than pure ambition:
MACBETH. ...I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other— (1.7.25-28)
Macbeth does, in fact, "o'erleap" himself, and fails in overcoming Malcolm and his own ambitions.