In this paper we report a case study of two first-year students at a university in Hong Kong doing the same writing assignment that required the use of sources. We explore the students' understanding of plagiarism, their strategies for composing, the similarity between their texts and source texts, and the lecturer's assessment of their work. The analyses in the study drew upon textual comparisons between student texts and source texts, interview data, and observation notes. The data indicated that both students appeared to understand the university's plagiarism policy yet their texts were characterized by patchwriting and inappropriate citation. Only one student's problems were spotted by the lecturer and checked with Turnitin while the other's was hidden to the lecturer. We speculate about the reasons, and then discuss these issues related to students' writing from sources: the place of reading in a source-based assignment, the difficulty level of sources for an assignment in an introductory course, complexities of attribution in source-based writing assignments, and the place of patchwriting in the work of novice writers. We conclude by highlighting the challenges faced by teachers and researchers and echo with others that different labels need to be given to plagiarism as cheating versus misuse of source texts. (Contains 1 table and 1 figure.)