Perhaps the most obvious incident foreshadowed by Santiago's comment that people have "forgotten to rely on their own instincts," in Paulo Coelho's The Alchemist can be seen at the story's end when Santiago is beset by thieves as he travels to the pyramids.
As Santiago travels across the desert, he comes to a sand dune, and there the omens tell him to dig. He digs on and on until his hands are scratched and he is exhausted. Suddenly, he hears footsteps behind him. The men tell Santiago that they have been displaced by the tribal wars. They are poor—thieves searching for money. Santiago is digging and they suspiciously want to know what he is looking for. Santiago is certain he has found his treasure (at last) and doesn't want to tell the men what he is doing.
...one of them seized the boy and yanked him back out of the hole. Another, who was searching the boy's bags, found the piece of gold.
"There's gold in here," he said.
This is all the others need to hear. One of the men grabs Santiago...
...and in the man's eyes the boy say death.
When the men demand that Santiago dig, he obeys. The men begin to beat him as dawn breaks—while he is bleeding, his clothes are torn and he is fearful that death is near, he admits to this captors that he is digging for treasure because he had had two dreams showing him treasure buried near the Egyptian pyramids.
The man who seems to be the leader of the group explains to his men that Santiago doesn't have any gold—he must have stolen what he had. He does not take any steps to kill the young man, nor do any of his men. But before the thieves leave, the leader comes to Santiago and tells him that he will live, but that he has been foolish. He tries to give the young man some advice. He explains that he once had a dream, but that following the images he saw in the dream would have been ridiculous.
I dreamed that I should travel to the fields of Spain and look for a ruined church where shepherds and their sheep slept. In my dream, there was a sycamore growing out of the ruins of the sacristy, and I was told that, if I dug at the roots of the sycamore, I would find a treasure. But I'm not so stupid as to cross and entire desert just because of a recurrent dream.
This man has forgotten to trust his instincts; he sees Santiago's actions as foolish; and he will never realize his Personal Legend. However, Santiago has not forgotten to trust his instincts. In pursuing his Personal Legend, following the omens and paying attention to his dreams, Santiago has come to this place. However, it is not to find his treasure, as he had thought. It is to speak to this thief and hear about his dream. For the dream of the thief will take Santiago back to his birthplace, to the very churchyard where his journey began as he cared for his sheep. In failing to trust his instincts, the thief provides Santiago with the information he needs to realize his dream.