Harmful friction is when the resistance of one body moving across another brings injury or undesirable results, according to Reference.com. All harmful friction does not necessarily involve injury.
Friction is desired in many instances, but in others, it brings unwanted results. For example, when baseball players slide into home plate, it produces harmful friction between their bodies and the ground that often results in painful scrapes. Harmful friction also occurs when a person wears a pair of ill-fitting shoes and they rub against his feet, forming a blister. Too much friction causes items to wear out and not work properly. Sometimes mechanical friction from parts in a machine or appliance causes an unwanted fire.