What flashback occurs in "To Build a Fire"?
As the man comes to severe difficulty after falling in the water and struggling to light a fire, he thinks back repeatedly to the old man at Sulphur Creek. He had had a conversation with them man before beginning his journey out to meet the boys and the man had suggested several things about his trip that he might think about before going.
As he falls further and further into grave danger, he thinks again and again that the old man was probably right. He ought to have not gone alone, he ought to have waited another day, etc.