Vitamin and mineral supplements are commonly used in diets for zoological and companion animals. Specific nutrient requirements are often unknown, so informed decisions are based on the literature for related species. Starting in November 2017, several cases of metastatic mineralization were detected in a population of earless lizards (Holbrookia lacerata) at the Fort Worth Zoo. Since then, 97% (32/33) of the H. lacerata population has died with 81% (26/32) of deaths submitted for histopathology. All submitted cases had mineralization in at least one tissue and 77% (20/26) were consistent with metastatic mineralization with no underlying cause detected histologically. The supplement dusted on the food items fed 5–6 times/wk for 2–4 mo was inadvertently switched and the incorrect supplement was found to contain four-fold the intended vitamin D3 concentration, approximately 20,000 IU/kg. Thus, hypervitaminosis D was considered the most likely cause. Interestingly, collared lizards (Crotaphytus collaris), also fed prey supplemented 6 times/wk, and 28 other insectivorous species possibly receiving the supplement 2–4 times/wk did not appear affected. During this time, only two other cases of metastatic mineralization were diagnosed in other herpetofauna at this institution. Prior to receiving the incorrect supplement, there were no cases of metastatic mineralization detected in the earless lizard population. These cases highlight species-specific sensitivities and how excess supplementation can have deleterious effects. It is important to confirm inventory at arrival, regularly conduct chemical analysis of supplements, and educate keepers and owners about supplements.