The Question and Answer section for To Build a Fire is a great resource to ask questions, find answers, and discuss the novel.
I think that there is a lot of foreshadowing in this story so I expected the man to falter. The Man's spit freezes before it hits the ground at the beginning of the story; the man steps through freezing water; the man builds an emergency fire...
The most difficult challenge presented by the Yukon at this time of year is the bitter cold. Another hazzard is the presence of thin ice.
Gradesaver has a complete summary and analysis readily available for the short-story, To Build a Fire. Simply follow the title link at the top of the page.