Our Gemara and Mishna on Amud Beis discusses a case of how annulment cannot work in a situation of mistaken identity:
מַתְנִי׳ נָדְרָה אִשְׁתּוֹ וְסָבוּר שֶׁנָּדְרָה בִּתּוֹ נָדְרָה בִּתּוֹ וְסָבוּר שֶׁנָּדְרָה אִשְׁתּוֹ …הֲרֵי זֶה יַחְזוֹר וְיָפֵר:
MISHNA: If a man’s wife took a vow and he thought that it was his daughter who had taken a vow, or if his daughter took a vow and he thought that it was his wife who had taken a vow…in each case, when he realizes his error with regard to the vow, he must repeat the action and nullify the vow a second time.
The Derashos HaRan (5) quotes this Mishna to raise a question as to how could have Yitschok’s blessing been redirected to Yaakov? If Yitschok had in mind his son Esav, as it seems he did, then the blessing should either sdtill go to Esav, or at least be non-valid, similar to the discussion in our Mishna?
The Ran explains that Yitschok’s blessings were not merely good wishes or prayers from Yitschok, as one might bless a person today. Rather, they came from a state of prophecy, and thus Yitschok’s intentions and thoughts were being directed in a way that was beyond his control. He later so much as says so to Esav, (Bereishis 27:33), “This one, too, shall be blessed”, meaning to say, “After the fact, I see that I was divinely directed to bless Yaakov.”
The Ran also makes another point. If Yitschok had such ruach hakodesh why was it even necessary for Yaakov to fool Yitschok? The Ran answers that indeed Yitschok loved Esav and was blind to his flaws. Had he been given divine privy to Esav’s ineligibility for the blessings due to his wicked propensities, Yitschok would have been devastated. Prophecy cannot occur in state of sadness or despair, it crushes the soul and distances a person from God. This is why Yitschok asks Esav to bring him delicacies and wine, so that he can delight and enter into state of spiritual ecstasy so that he could bring down the Divine blessings. Ironically, if Yitschok found out prior to the blessings that Esav was disqualified, his heartbreak would disrupt his ability to connect to God, and he would not have been able to give anyone a blessing, Yaakov or Esav.
One might ask, could not God have given Yitschok the sense of joy, even so? The Ran does not say this, but I would answer based on an important principle stated by the Rambam in Guide for the Perplexed (III:32). God does not interfere with human thoughts. Even when rendering miracles that defy nature, God always leaves humans in full control of their own thoughts. This is why the verse states (Shemos 13:7) God did not guide the Jewish people to Canaan via the direct route through the land of the Philistines, “Lest the people relent upon encountering warfare and run back to Egypt.” According to the Rambam, it would be against God’s rule book to have given the Jewish people courage by inserting bravery into their heads; God could only lead them physically in ways that they could, through their own recognition, come to a place of courage or faith. So too, God would not make Yitschok happy by inserting happy thoughts into his head.
This is important to know. God will not interfere with our thoughts, be they good or evil. We must be allowed our autonomy in such a personal area.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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