I carry a set of bearings & seals. My entire career of 40 years was about bearings, maint. & proper care. Things do happen, but there is always a cause of failure and 99.999 percent of the time it's something other than the bearing. Moisture, dirt, too little lubricant, too much, wrong type, too much installation preload, too little, overloading, improper storage, damage prior to installation(it was dropped, greased with dirty hands, etc) - these are just a few of the causes, so just in case I carry that set, but cause is in general, mishandling to begin with.