I don't entirely agree with your assessment. To a certain extent, his decision to go into the Alaskan bush with only minimal, inexpensive equipment was apparently a conscious decision, not an oversight. I might disagree with it, but I can't really call it a "mistake" any more than Reinhold Meissner made a "mistake" climbing Mt. Everest without oxygen. I would be careful about labelling something a "mistake" just because I personally wouldn't do it.

However, having said that, I felt that McCandless was arrogant and hubristic - i.e. he thought he knew it all, and he had mostly contempt for local knowledge. He also didn't seem to act with a great deal of common sense.

For example, I can understand - maybe even sympathise - with his decision not to bring a map of the area. I think Krakauer hit the nail on the head here; the only way to simulate exploring an uncharted wilderness in this day and age is to leave the map at home. Others have explored uncharted territory before; why shouldn't he be able to do the same? But having left the map at home, the next logical step would be to develop your own - spend part of each day, at least, exploring the area to see what resources it had to offer. McCandless, as far as anyone can tell, was completely unaware of the cable-car crossing the river only 5 miles from his camp - even in his weakened condition, he could probably have crossed the river and hiked to safety. But in the several months he spent in the area, he apparently never attempted to do this.

If he learned from his mistakes at all, then he seems to have done so one mistake at a time. For example, if you camp in a flood plain and get washed out by a flash flood, one lesson you can learn is not to camp in a flood plain. Another lesson you can learn is to familiarise yourself with local hazards before you go adventuring there. McCandless seems the type who would have learned the former lesson but not the latter; ie he couldn't generalise his lessons learned to other, seemingly dissimilar, situations.

Another aspect of his character seemed to be that, every time he survived one of his foolhardy stunts, he interpreted that as proof of his superiority, rather than a warning signal. It's like someone who plays Russian Roulette once a month and, having survived three times, concludes that he's exceptionally skilled at Russian Roulette (or else that it's not as dangerous as most people make it out to be). For many people, such experiences would tell them they were living on borrowed time; for McCandless, it seems to have reinforced his conviction that he could improvise his way out of any situation.