Developing small learner and native corpora, this case study examines how Korean L2 learners used six types of lexical collocations in L2 writing to address (a) the frequency and acceptability of learner collocations, (b) problematic constituents of deviant collocations, and (c) possible sources of the learner difficulties. The overall frequency (about 8% of each corpus) and relative frequencies of each collocation type were similar between the learner and native corpora in descending order of adjective-noun, verb-noun, noun-noun, adverb-verb, adverb-adjective, and noun-verb combinations. The average and individual acceptability rates of each collocation type were around 70% and the problematic constituents were found both in nodes and collocates. L2 influence on learner difficulties mostly lied in confusions about synonyms, overuse of delexical verbs, and use of correct collocations in wrong contexts. Relying on L1 semantic representations, the learners produced non-habitual combinations, misrepresented the intended meaning, and paraphrased L2 collocations. Pedagogical implications arose for teaching L2 collocations about the importance of considering the immediate context of L2 writing and taking different approaches to different types of collocations.