I've run across something similar recently.

One thing that seems counter-intuitive is that you could have a  
method somewhere like:

makeC3

	^C3 new

This method has been compiled and the reference to class C3 has been  
resolved in the compiled code.  If the C3 class has gone away - the  
class is renamed to AnObsoleteC3.  So the code appears to reference  
C3, but it really references the obsolete one.  You need to recompile  
this method - just trivially alter it by adding a space and accept  
it.  The method will be recompiled.  You might want to do a source  
code search for C3, then ensure that all those methods get recompiled.

-Todd Blanchard