Despite the efforts exerted on error correction by teachers and students during English for Academic Purposes (EAP) courses, the literature is split about whether such instruction succeeds in reducing grammatical and lexical errors in student writing. In this study, a longitudinal corpus of essays and reports collected at three key data points along a tertiary EAP course (pre-course, immediate post-course and final assessment) totalling 205,682 words was annotated for 12,996 errors across 10 grammatical and lexical error types. The results, including a mixed-effects linear model, showed a general significant decline in the number of errors produced over the duration of the course. However, closer examination revealed that the frequencies of the majority of individual lexical and grammatical error categories remained unchanged over time, despite specific out-of-class instruction on these errors as well as numerous occasions where teachers provided written corrective feedback. The overall usefulness of the error correction feedback and instruction resulting from the EAP course in question therefore remains inconclusive, although students and other stakeholders may still call for the inclusion of such feedback and instruction on EAP curricula.