The goal of this paper is to audit null-annotated parent-child pairs in the UMLS Metathesaurus. We have developed techniques for identifying suspicious pairs with high likelihood of errors by using inconsistencies between the hierarchical relationships of the Metathesaurus and the Semantic Network. Two formal conditions, called semantic inversion and lack of ancestry are investigated. Analyzing two corresponding samples shows that semantic inversion is significantly more likely to indicate an error than lack of ancestry, which in turn is more likely to indicate errors than a consistent configuration. We also discuss cases of parent-child pairs with semantic inversion that may be corrected by disambiguating the child.