I've noticed a problem with Smart Slug 1.3 where the default stop words don't get removed when stopwords_set is specified in the plugin settings. This is the default setting, meaning that by default the plugin doesn't remove the default stopwords. I don't think this is correct behavior.
You can work around this by finding the stopwords_set list in plugins/smart-slug/wet_smartslug-en_EN.po and pasting it into the stopwords settings.
Any idea what might be going wrong here?