This study analyzes common issues in the writing of our upperyear, undergraduate computer science students in timed (e.g. tests) and untimed (e.g. longer assignment reports) scenarios. Our goal is to identify writing issues that should be addressed earlier in the CS curriculum. In collaboration with a writing specialist, we develop and fine-tune a rubric with Grammar, Conciseness, Clarity, Organization, Structure, and Formality as the main categories.We find, in our writing samples, that grammatical issues (such as punctuation errors or run-on sentences) are common even in a setting where students have ample time to proofread. Other common issues include unclear pronoun antecedents, lacking topic sentences, and other structural and organizational issues. Moreover, we correlate the presence of these issues with the students' grades. We do not find statistically significant correlations between writing skill and grades, and the weak correlations we find depend heavily on the writing context.We hope that our rubric items and findings are useful for instructors in targeting writing skills that warrant development.