Research and pedagogical materials focusing on the acquisition of the determiner/quantifier system in English frequently highlight countability as a locus of difficulty for English language learners, particularly learners whose languages do not overtly mark a count-mass distinction. The current study uses a document analysis approach to analyze unedited written essays from the ICNALE corpus, produced by 52 Mandarin Chinese writers of L2 English across four proficiency levels. A target language use (TLU) analysis of the essays, all written on the same topic, suggests that prototypically count and mass nouns are not a source of inherent difficulty for learners. Instead, count-mass flexible nouns are the locus of the greatest number of errors, due to an inability to use these nouns correctly in context. In this article, I argue that a reanalysis of ESL/EFL pedagogical practice is necessary, one that focuses specifically on count-mass flexible nouns, and revisits determiner (especially article) use whenever new vocabulary is introduced. Presenting flexible countability as one facet of a noun's semantic meaning, I argue, will add lexical depth while concurrently allowing for more target-like use of determiners and quantifiers.