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Variability models, feature diagrams ahead, have become commonplace in the software product lines engineering literature. Whereas ongoing research keeps improving their expressiveness, formalisation and automation, more experience reports on their usage in real projects are needed. This paper describes some challenges encountered during the re-engineering of PloneMeeting, an Open Source software family, into a software product line. The main challenging issues we could observe were (i) the ambiguity originating from implicit information (missing definitions of feature labels and unclear modelling viewpoint), (ii) the necessity of representing spurious features, (iii) the difficulty of making diagrams and constraints resistant to change, and (iv) the risks of using feature attributes to represent large sets of subfeatures. Our study reveals the limitations of current constructs, and calls for both language and methodological improvements. It also suggests further comparative evaluations of modelling alternatives.