If a mouse fails to operate or should it perform erratically, it may need a ball replacement. Mouse balls are now available as FRU (Field Replacement Units). Because of the delicate nature of this procedure, replacement of mouse balls should only be attempted by properly trained personnel.
Before proceeding, determine the type of mouse balls by examining the underside of the mouse. Domestic balls will be larger and harder than foreign balls. Ball removal procedures differ depending upon the manufacturer of the mouse. Foreign balls can be replaced using the pop off method. Domestic balls are replaced by using the twist off method. Mouse balls are not usually static sensitive. However, excessive handling can result in sudden discharge. Upon completion of ball replacement, the mouse may be used immediately.
It is recommended that each person have a pair of spare balls for maintaining optimum customer satisfaction. Any customer missing his balls should contact the local personnel in charge of removing and replacing these necessary items.
Please keep in mind that a customer without properly working balls is an unhappy customer.
According to Snopes, this has been doing the rounds since 1989, and indeed I remember a giggling veterinary student sharing it with me around then.