Efforts to promote disciplinary literacy can help students integrate knowledge with ways of doing and being within disciplinary settings. Yet, effectively facilitating disciplinary literacy, even within an upper-level undergraduate physics course like the one studied here, is surprisingly hard. This article qualitatively analyzes an instructor's responses to student lab reports and finds that his comments to students focused on issues of correctness, often at the expense of larger rhetorical concerns of the text. Analysis also suggests that the instructor was thinking about many rhetorical aspects beyond surface-level errors as he read. Together, these findings suggest that efforts to promote disciplinary literacy, especially related to writing instruction, benefit from recognizing the layered contexts of activity in which writing and responding to lab reports take place. These findings hold value for secondary and postsecondary literacy instruction; in broad terms, this study may serve as a cautionary tale by illuminating the overlapping and competing value systems involved in disciplinary literacy efforts.