Colin Putney uploaded a new version of System to project The Trunk:
http://source.squeak.org/trunk/System-cwp.467.mcz

==================== Summary ====================

Name: System-cwp.467
Author: cwp
Time: 28 December 2011, 10:43:22.531 pm
UUID: f047d655-bc2b-4ea1-a7a8-c091bac02b0b
Ancestors: System-laza.466

SystemOrganizer>>classify:under: now uses #at:ifAbsent: to look up a class when generating system change notifications. This prevents bogus change notifications and walk backs when running tests against SystemOrganizer instances.

=============== Diff against System-laza.466 ===============

Item was changed:
  ----- Method: SystemOrganizer>>classify:under: (in category 'accessing') -----
  classify: element under: newCategory
+ 	| oldCategory class |
- 	| oldCategory |
  	oldCategory := self categoryOfElement: element.
  	super classify: element under: newCategory.
+ 	class := Smalltalk at: element ifAbsent: [^ self].
  	SystemChangeNotifier uniqueInstance
+ 		class: class
- 		class: (Smalltalk at: element)
  		recategorizedFrom: oldCategory
  		to: newCategory !