Over two decades ago, a primigravid female presented with concern for recurrence of an adverse phenotype affecting her three brothers. The three brothers presented with intellectual disability, developmental delay, behavior problems and dysmorphic features. The screening tools available at the time revealed an FGD1 variant present in all three brothers, their mother being a carrier, absent in their unaffected uncle, and absent in the proband herself. This variant was hypothesized to be explanatory, but years later more advanced genetic screening showed that it was benign. Episign analysis revealed the true cause, a novel pathogenic KDM5C variant. This case study provides further insight into the KDM5C phenotype and demonstrates the importance of amending past errors as science evolves.