As far as I know, most of the programs and teaching materials for L2 learners do not incorporate their specific difficulties in a systematic way.One reason for this seems to be the lack of consistent descriptions of learners’ grammatical knowledge in the different stages of their linguistic development. This study illustrates the potential of linguistic research for the identification and description of “resistant” grammatical features and rules captured in written productions of Portuguese L2 advanced learners. of interlanguage grammars, mentioning briefly some taxonomies of errors of Portuguese L2 already available. Afterwards, taking a written corpus produced by Mozambican university students as the empirical basis, I present two case studies which show how a deeper analysis of the errors makes it possible not only narrowing their scope, but also providing more consistent data for planning teaching of Portuguese L2 to these advanced learners. Finally, I alert to the limits faced when analyzing some types of errors in formal education settings in multilingual communities where new varieties of Portuguese are emergin