This paper describes an extension to the MOSAIC model which increases MOSAIC’s fit to the cross-linguistic occurrence of Optional Infinitive (OI) errors. Previous versions of MOSAIC have successfully simulated OI errors as structures with missing modals or auxiliaries, but have underestimated their frequency in some languages. Here, we explore defaulting effects, where the most frequent form of a given verb is substituted for less frequent forms, as an additional source of OI-errors. Defaulting in English tends to result in the production of bare forms that are indistinguishable from the infinitive. In Spanish it is less pronounced, and tends to involve the production of 3rd person singular forms. Dutch verb forms are dominated by the stem in corpus-wide statistics, and the infinitive in utterance-final position, suggesting qualitative change in defaulting across development. Defaulting is argued to have the potential to simulate the cross-linguistic pattern of verb-marking errors in children with SLI.