The present article addresses the issue of second language accuracy developmental trajectories and shows how they can be captured via an error-tagged version of an English as a Foreign Language (EFL) learner corpus. The data used in this study were extracted from the International Corpus of Learner English (Granger et al., 2009) and consist of a total of 223 learner essays. Each composition was (a) manually and exhaustively annotated for errors following the Louvain error-tagging taxonomy (Dagneaux, Denness, & Granger, 1998) and (b) individually rated by two (sometimes three) testing experts according to the Common European Framework of Reference for Languages (Council of Europe, 2001) descriptors for linguistic competence. As a result, each text was assigned a B1 (lower intermediate), B2 (upper intermediate), C1 (advanced), or C2 (near-native) score. A refined counting method, potential occasion analysis, which relies on both an error-tagged and a part-of-speech-tagged version of the learner data, was used to quantify the errors. This allowed the analysis to trace the type of development displayed by more than 40 error types along the B1--C2 proficiency range. The results indicate that the EFL error developmental patterns tend to be dominated by progress and stabilization trends and that progress is often located between B1 and B2. (Contains 18 tables, 6 figures, and 9 notes.)