Grammatical constraints impose diverse requirements on the relations between words and phrases in a sentence. Research on the online implementation of grammatical constraints reveals a strikingly uneven profile. The parser shows impressive accuracy in the application of some rather complex constraints, but makes many errors in the implementation of some relatively simple constraints. Just as the study of optical illusions has played an important role in the study of visual perception, the parser's highly selective vulnerability to interference and "grammatical illusions" provides a valuable tool for understanding how speakers encode and navigate complex linguistic representations in real time.