The mistakes and various miscalculations are listed mostly in Chapter 17 of this excellent book, which details the visit that the author made himself to see where Chris McCandless lived his last days and where he sought to live off of the land by himself. He details how he was able to find a way of crossing the river that was so powerful when Chris tried to leave the wilderness, stating that a way of crossing was clearly detailed on the map that he had with him. Chris McCandless, however, because he wanted to dispense with Western civilisation, did not take a map with him and therefore cut himself off from finding an escape route from his self-imposed prison. This was not the only mistake or miscalculation he made, as the following quote demonstrates:
By design McCandless came into the country with insufficient provisions, and he lacked certain pieces of equipment deemed essential by many Alaskans: a large-calibre rifle, map and compass, an ax.
The author comments that this is indicative of not only stupidity but of massive arrogance that eventually cost Chris McCandless his life. Whilst the author does give massive credit to Chris McCandless for being able to survive for a long period of time in the wilderness by his wits alone, he also does mention the fact that he had not mastered enough of the survival skills necessary to live off of the land, such as his killing of the caribou evinces.