While the field of grammatical error detection has progressed over the past few years, one area of particular difficulty for both native and non-native learners of English, comma placement, has been largely ignored. We present a system for comma error correction in English, which achieves an average of 89% precision and 25% recall on two corpora of unedited student essays. This system also achieves state-of-the-art performance in the sister task of restoring commas in well-formed text. For both tasks, we show that the use of novel features which encode long-distance information improves upon the more lexically-driven features used in prior work.