Non-adult-like linguistic behavior in children is sometimes taken as evidence for endogenous factors that drive selection of grammatical features from the child's hypothesis space of possible grammars. Analyses of English-acquiring children's productions of medial "wh"-phrases exemplify this trend in particular. We provide an alternative account of these productions as performance errors arising from underdeveloped cognitive inhibition. We offer experimental evidence in favor of our failure of inhibition account. The results argue against treating these errors as reflecting incomplete or non-target acquisition of grammatical features. Instead, the results support a theory of how these errors arise and are subsequently purged from children's productions that reduces to a theory of how cognitive inhibition develops during childhood.