"Memo of the Month," From _The Washington Monthly_, January/February 1991, 
page 24:
This is an actual alert to IBM Field Engineers that went out to all IBM 
Branch Offices.  The person who wrote it was very serious.  The rest of us 
guys find it rather funny.
Abstract:  Mouse Balls Available as FRU (Field Replacement Unit)
Mouse balls are now available as FRU.  Therefore, if a mouse fails to 
operate or should it perform erratically, it may need a ball replacement. 
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 manufacturer 
of the mouse.  Foreign balls can be replaced using the pop-off method. 
Domestic balls are replaced 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 replacer have a pair of spare balls for 
maintaining optimum customer satisfaction, and that any customer missing his 
balls should suspect local personnel of removing these necessary items. 
To re-order, specify one of the following: 
P/N 33F8462 - Domestic Mouse Balls
P/N 33F8461 - Foreign Mouse Balls