Dental calculus and other preserved microbiome substrates are an attractive target for dietary reconstruction in past populations through a variety of physical, chemical, and molecular means. Recently, studies have attempted to reconstruct diet from archaeological dental calculus using archaeogenetic techniques. While dental calculus may provide a relatively stable environment for DNA preservation, the detection of plants and animals possibly consumed by an individual through DNA analysis is primarily hindered by microbial richness and incomplete reference databases. Moreover, high genomic similarity within eukaryotic groups - such as mammals - can obfuscate precise taxonomic identification. In the current study we demonstrate the challenges associated with accurate taxonomic identification and authentication of dietary taxa in ancient DNA data using both synthetic and ancient dental calculus datasets. We highlight common errors and sources of contamination across ancient DNA datasets, provide recommendations for dietary DNA validation, and call for caution in the interpretation of diet from dental calculus and other archaeological microbiome substrates.