I agree with Blissful that novice backpackers are much more likely to pack too heavy than to pack too lightly. I would also argue that the assortment of items in the pack is rarely the cause of really bad experiences--it's the cascade of poor decisions of all sorts, made by people so lacking in knowledge that they don't even recognize how ignorant they are, and blithely head into situations from which no gear would be able to rescue them.