Particularly in dynamic applications, abrasion occurs due to contact between the O-ring and a harder mating surface. This continuous friction causes the O-ring to gradually lose mass.
In the long term, this causes the O-ring to lose so much mass that the O-ring becomes leaky and has to be replaced. As a basic rule, dynamically loaded O-rings should be replaced much more frequently than statically loaded O-rings.
It should also be noted that the abrasion of the O-ring can contaminate the medium. This is particularly important if the medium is a food product (e.g. milk).