Children's reported copying-without-deletion errors, like ‘Whose is that is?’, have often been interpreted as the result of a non-adult rule of subject–aux inversion. By contrast, this study presents a performance account of these errors and supports this account by investigating what factors impede children's performance. Using an elicitation task, 16 test sentences were evoked from 16 3- to 5-year-old children. In particular, errors appeared (1) when the subject NP contained a relative clause, (2) when the relative clause had an object gap, and (3) when the relative clause was long. Since errors occurred in response to some sentence types and not others, and the children who made copying-without-deletion errors produced at least four correct responses, these results were interpreted as support for the performance account.