The study explores the causes of attraction errors by combining a verb selection task and a comprehension task. Results show that although more verb selection errors occur with number mismatch between the agreement head and the attractor, more comprehension errors occur with number match. Finer analyses suggest that agreement errors arise both because of incorrect structure building (sometimes, the wrong NP controls agreement) and because of feature contamination (the distractor’s feature is transferred, though the correct structure is built), a view which is compatible with two principles of self-organized sentence processing: similarity-based interference at encoding and feature passing.