The big mistake that the man makes is that He builds his fire under a spruce tree.
Explanation:
In “To Build a Fire” by Jack London. the character of the man, directs himself to a fatal ending because he didn't listen to the warnings of his surroundings, then near to his inevitable death he tries to save himself by building a fire but he does it under a spruce and all the snow that was above it falls down on his fire extinguishing it and making him frozen to dead.
