I have only a minor refinement to John Sarkela 's excellent analysis of the 
use of exceptions in this situation:

   Because of the propensity of people to write;
	 [...] on: Error do:...

I would make CollectionError a refinement of Notification rather than 
Error.  In addition, I would be inclined to name it QuestionableUsage (or 
QuestionableCollectionUsage if you feel the need to be more specific.)

Regarding, the ANSI spec. stating that Set>>add: is undefined for nil 
arguments.  There are at least two different reasons why that choice may 
have been made (and I don't recall which was the actual reason):
	1)  Different existing (in 1998) implementations did different things
	2)  All existing implementation did the same thing but the committee had 
reservations about that definition and wanted to leave room for future 
implementations to do something different.

Allen_Wirfs-Brock at Instantiations.com