I assume you are refering to the comment Lady Macbeth makes in Act II scene 2 when she comments that she would have killed Duncan herself "had he not resembled" her father as he slept. What is surprising about this comment is the way in which we see a crack in the otherwise determined and resolute face of Lady Macbeth. Let us remember that from her very first introduction to us in Act I, she has shown herself to be ruthlessly committed to doing anything necessary in order to make the prophecy of the witches come true, even going as far as abandoning herself completely to the forces of evil. Therefore, I would argue that her weakness shown here is definitely not expected, and it certainly takes us by surprise.
As to how we can account for it, perhaps this weakness is just an indication that Lady Macbeth is normal and human just like the rest of us. It certainly does foreshadow her mental breakdown and instability when she begins sleepwalking and is unable to put the past behind her. Macbeth is showing us the consequences of placing ourselves so firmly in the grip of evil.