The slot-unbound standard generic function is invoked when an attempt to read an unbound, but existing slot in an instance is made (if this explanation is unclear, please review the earlier article on slot-boundp).  Once more, this function is not for the programmer to call into, but by specializing this function it is possible to take special action when an attempt is made to read the contents of an unbound slot.

In effect, slot-unbound has the same relationship to slot-boundp that slot-missing has to slot-exists-p.