This paper aims to dissociate grammatical and general cognitive (e.g., working-memory based) accounts of the processing costs elicited by word order variations in German. To this end, we present a study using event-related brain potentials (ERPs), in which dislocated arguments were instantiated either by non-pronominal or by pronominal noun phrases. This manipulation allows for a dissociation of the two competing accounts, since only a dislocation of non-pronominal arguments gives rise to a non-canonical structure. The results show that (a) in sentences with non-pronominal arguments, the determiner of a non-canonical noun phrase elicited a broadly distributed negativity, and (b) in sentences with pronominal arguments, no differences were observed for object- vs. subject-initial word orders. These findings show that the human parser is sensitive to fine-grained grammatical regularities. We therefore argue that the negativity is a reflection of a local syntactic mismatch, rather than of an increase in working-memory load.