Ontology evaluation is one of the most important phases of Ontology Engineering. Researchers have identified different types of errors that should be catered in ontology evaluation process and classified them in error's taxonomy. Semantic Inconsistency Errors are the most common when modeling complex taxonomic knowledge while building semantic classification within the ontology, and in ontology merging process where different subtype concepts of source ontologies map on each other. In this paper, we have identified three types of semantic inconsistency errors, and provided criteria for semantic classification evaluation. This criterion provides ontologists to build well-formed class hierarchy that is free from semantic inconsistency errors. Moreover we emphasized on the control mechanisms (based on this criteria) to be perform during finding mappings between concepts in ontology merging process to achieve accurate results. We have demonstrated the importance of such errors by giving different scenarios where appropriate.