The answer to your question depends on who the consequence is "unforeseen" by: Macbeth, other characters, or the audience. I'll just give you an example or two and let you apply them to whatever you need.
The first scene in Act 3 involves Banquo revealing that he suspects Macbeth of treachery. This may have been unforseen by Macbeth, although he certainly knows Banquo has the potential do to him great harm, since Banquo knows about the witches' predictions. Also, Macbeth's treachery was certainly unforseen earlier in the play by Banquo.
Thou hast it now--King, Cawdor, Glamis, all
As the Weird Women promised, and I fear
Thou played'st most foully for't. (3.1.1-3)
Another example of an unforeseen consequence in Act 3 is in the same scene, only a few minutes later. Macbeth uses irony to deceive Banquo, to protect himself from accusation, and to gain information from Banquo so he can set a trap to murder him. This is unforeseen, not because Macbeth plans to kill Banquo, but because he does it so matter-of-factly, and seemingly without any guilt. He suffers great guilt before and immediately after killing King Duncan, but he seems to no longer possess any scruples whatsoever.
We should have else desired your good advice,
Which still hath been both grave and prosperous,
In this day's council [Macbeth would have liked to have had time to talk to Banquo today]; but we'll take tomorrow [Macbeth will settle for talking to him tomorrow--verbal irony because he knows Banquo will be dead tomorrow, since he has arranged for killers to murder him tonight].
Is't far you ride? (Act 3.1.21-24)
When Macbeth talks about seeing Banquo tomorrow, he is providing evidence, for later, that he did not know Banquo was going to be murdered, since others are present during this conversation.
When Macbeth asks how far Banquo is riding, he is seeking the necessary information to set up a time line for when he can set the trap.