This essay is about a sparsely known public, but who does exist in the field of the didactics of French for foreigners, despite of its invisibility among the institutions. It is a range of learners with a multiple statut : minor, foreigner, isolated, but above all, learner, designation often forgotten. Through their situation of mobility, these learners group together several specificities, in which two major phenomenons, labelled as perturbators in didactics : the heterogeneity and the unexepted events. The heterogeneity in didactics designates the differences that remain among the learners. It is seen as disruptive because it does not allow the unification of the group. The unexpected event, is an event that pops out where we do not expect it. To resign ourselves in front of these two phenomenons would be the easiest solution, because rethink our beliefs and actions is not easy, even more when our University courses do not always correspond to reality.