Purpose: This study aimed to compare sound production errors arising due to phonological processing impairment with errors arising due to motor speech impairment. Method: Two speakers with similar clinical profiles who produced similar consonant cluster simplification errors were examined using a repetition task. We compared both overall accuracy and acoustic details of hundreds of productions with target consonant clusters to tokens with singletons. Changes in accuracy over the course of the

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... udy were also compared. Results: In target words with consonant cluster simplification, the individual whose errors reflected phonological impairment produced articulatory timing consistent with singleton onsets. These productions improved when resyllabification was possible, but error rates were not affected by exposure. In contrast, the individual with motoric-based errors produced simplifications that contained the articulatory timing associated with clusters. Accuracy was not affected by the ability to resyllabify, but it did significantly improve following repeated production. Conclusions: Our findings reveal clear differences between errors arising in phonological processing and in motor planning that reflect the underlying systems. The changes over the course of the study suggest that error types with different sources are responsive to different intervention strategies.