This article documents the typological occurrence and interactions of two seemingly independent error patterns, namely Velar Fronting and Labial Harmony, in a cross-sectional investigation of the sound systems of 235 children with phonological delays (ages 3;0 to 7;9). The results revealed that the occurrence of Labial Harmony depends on the occurrence of Velar Fronting, and that, when these processes co-occurred, all three predicted types of interactions were attested. A constrained version of Optimality Theory is put forward that offers a unified explanation for the implicational relationship between these error patterns and their observed interactions. The findings are compared with the results from other studies and are considered for their theoretical and clinical implications. (Contains 1 note.)