A load that does not rotate for a long period of time (a car that has not been moved for several months, for example) can cause ball bearing failure. Minute vibrations and gravity cause lubricants to puddle at the lowest point, creating dry spots. Absence of lubrication in these spots creates points of metal-on-metal contact that can cause bearing failure once rolling resumes.
Scratching, nicking or denting can occur when the outer surface of the ball bearing is compromised by contaminants within the races (the casing in which the balls sit).
In metallurgy, fatigue refers to the wear metal undergoes when subjected to repeated opposing stresses. An example is the way a paperclip will break with repeated back-and-forth bending. Any point of contact between the ball bearings and the races creates a risk of unit failure from fatigue.
Pressure welding occurs when two pieces of metal are forced together under pressure so intense it melts the metals, causing them to weld. Microscopic irregularities in ball bearings push off lubricants, creating spots of metal-on-metal contact where friction can produce heat of this intensity. If the bearing continues to roll, these microscopic welds are torn off. The effect can be threefold: the bearing and race can become increasingly irregular, creating increased pressure welding/tearing and eventual unit failure; friction can overwhelm the force attempting to turn the load over the bearings and the unit will seize; or the unit becomes contaminated with pieces of torn off metal, creating abrasions and eventual failure.