It didn't halt on your machine because you didn't copy and paste the code that halts (i guess I wrote too much code in the mail so it was not clear). You copied the code that I explicitly said that doesn't halt because the error is raised at compilation time.
This code halts (copy, paste and run it you will see):
[ OpalCompiler new evaluate: 'AnUnknownClass subclass: #Adaptor1Example
� � � � instanceVariableNames: ''customers accountID address name phoneNumber''
� � � � classVariableNames: ''''
� � � � poolDictionaries: ''''
� � � � category: ''Examples-Cookbook''' ] on: OCSemanticWarning do: [ :e | e halt ]
We can discuss this another day when you have time because seemingly my mail was not clear (you said you couldn't trap the semantic warning). Now I'm really sick today so we can discuss friday or next week.