Importoldchangelog plugin tries to disable itself when it has finished importing and converting the old changes.log's content by calling plugin_disable. This works only if lib/plugins/importoldchangelog is writable by the web server and this is not verified. Thus, importoldchangelog plugin stays enabled after its action (do=check shows that).
I wonder if there is anything to do. do=check shows a "Importoldchangelog plugin not disabled after import" and there can't be a lot of reasons why the plugin could not disable itself. plugin_disable() could do a few tests and output a more explicit message (such as lib/plugins/importoldchangelog is not writable). Well, do we really need that? :-)