Morphological variability and the source of these errors have been intensely debated in SLA. A recurrent finding is that postpuberty second language (L2) learners often omit or use the wrong affix for nominal and verbal inflections in oral production but less so in written tasks. According to the missing surface inflection hypothesis, L2 learners have intact functional projections, but errors stem from problems during production only (a mapping or processing deficit). This article shows that morphological variability is also characteristic of heritage speakers (early bilinguals of ethnic minority languages) who were exposed to the family language naturalistically in early childhood but failed to acquire age-appropriate linguistic competence in the language. However, because errors in heritage speakers are more frequent in written than in oral tasks, the missing surface inflection hypothesis does not apply to them. The discussion considers how morphological errors in the two populations seem to be related to the type of experience.