Individual users of English as a first or second language are assumed to possess or aspire to a monolithic grammar, an internally consistent set of rules which represents the idealized norms or conventions of native speakers. This position reflects a deficit view of L2 learning and usage, and is at odds with usage-based approaches to language development and research findings on idiolectal variation. This study problematizes the assumption of monolithic ontologies of grammar for TESOL by exploring a fragment of genre-specific lexico-grammatical knowledge (the can you/could you V construction alternation in requests) in a single non-native user of English, post-instruction. A corpus sample of the individual's output was compared with the input he was exposed to and broader norms for the genre. The analysis confirms findings in usage-based linguistics which demonstrate that an individual's lexico-grammatical knowledge constitutes an inventory of constructions shaped in large part by distributional patterns in the input. But it also provides evidence for idiosyncratic preferences resulting from exemplar-based inertia in production, suggesting that input is not the sole factor. Results are discussed in the context of a "plurilithic" ontology of grammar and the challenges this represents for pedagogy and teacher development.