Purpose: This study describes a post hoc analysis of segmental, stress, and syllabification errors in third graders' productions of derived English words with the stress-changing suffixes "-ity" and "-ic." We investigated whether (a) derived word frequency influences error patterns, (b) stress and syllabification errors always co-occur, and (c) derived word stress pattern affects errors. Method: A total of 1,900 productions from 81 third-grade children were transcribed and coded. The targets were 8 high-frequency (HF) and 8 low-frequency (LF) real English words and 8 nonsense (NS) derived words. Participants combined a suffix and a base word to produce a derived word. The vowel-initial suffixes required both stress and syllabic changes in a base word. Results: Children made more segmental and stress errors on NS words than on HF and LF words, but more syllabification errors on LF words than on HF and NS words. More items among the LF words required vowel alternations than among the HF words. When syllabification was not mastered, stress was usually incorrect; however, when syllabification was correct, children often still erred on stress. Derived words that contained 2 trochaic feet were produced with fewer errors than were those that only had 1 trochaic foot. Conclusion: Rather than a frequency effect, a lexical effect emerged such that real words were more similar to each other for segmental and stress errors than they were to NS words. Three findings suggest that prosody plays a role in errors: Children made more suprasegmental than segmental errors, they appeared to master syllabification before stress placement, and they produced fewer errors on derived words with 2 trochaic feet than on those with only 1 trochaic foot. Although these results are preliminary, they highlight the role of prosody in morphophonology.