Mouse balls and mouse ball inspector

I don't know how they wrote this with a straight face. This was a
real memo sent out by IBM to its employees in all seriousness. It went
to all field engineers about a computer peripheral problem. The author
of this memo was quite genuine. The engineers rolled on the floor!
Especially note the last couple of sentences.

"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,
a 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."