This article examines alternating verbs (such as "quemar"(se) "to burn") in second language (L2) Spanish by considering the learnability problem from a sequence learning perspective (N. Ellis, 1996, 2002). In Spanish, verbs of the alternating class are obligatorily marked with the clitic "se" in their intransitive form. Errors of omission among English-speaking learners, who transfer zero-derived morphology from their native language, have been previously documented (Montrul, 2000). This study also examines a different kind of error--overgeneralization of "se" to transitive event scenes--that is hypothesized to result from chunking "se" with particular lexical items. The results of a picture description task reveal that learners frequently make this type of overgeneralization error, but that they are able to recover from it at more advanced levels of proficiency. These findings suggest that the acquisition of L2 morphosyntax is shaped by learners' familiarity with individual lexical items and the sequences in which they tend to appear.