This shows that there is a BC break as you don't change the code and the exception class changes.. And because of this, I don't think we should make this change. The only option to avoid BC would be to make MongoDuplicateKeyException inherit from MongoWriteConcernException. But in that case, we should have a chat about exception hierarchies and figure out whether we really really want a different exception class for each different error the server can throw... that'd mean hunderds of them :-S