We examined ERP correlates of L1 attrition and L2 processing. We tested lexical-semantic processing of confusable Italian words in sentences. Proficiency (L1 and L2) modulated N400 responses to errors involving confusable nouns. Attriters showed an enhanced P600 compared to non-attriting native-speakers. The P600 for lexical-semantic anomalies may reflect increased conflict-monitoring in attriters.