Shoe failure on a long trip is a possibility. I actually buy new shoes quite often. I take neither new shoes nor old shoes on longer trips. You never know if your new shoe is a lemon- manufacturing flaws usually show up quickly. One trip my hiking partner's shoe sole was coming apart. We were in an area where horsepackers frequent, so we searched many sites and found some wire. He wrapped the wire around his shoe and it lasted the rest of the trip! In the old days when we used boots, we would take boot nails with us. New boots are now just glued and I am not sure a boot nail would work on light hikers. BrianLee is right. Once a shoe starts for fail, you have limited miles left.