Here, we present a case of a 2.5-month-old infant presenting with failure to thrive. The clinical presentations were nasal blockage, noisy breathing, poor feeding, lethargy, dehydration, and fever for 3 days with decreased appetite. Her urine culture report showed a urinary tract infection. The biochemical evaluation demonstrated raised total serum calcium (6.0mmol/l) and serum 25-hydroxy vitamin D (>160ng/ml) with suppressed parathyroid hormone concentration (3.7pg/ml), which was the major concern to the clinicians. On ultrasonographical examination, nephrocalcinosis was observed. Further evaluation unveiled that the vitamin D supplement administered to the infant constituted a deucedly high dose of 42000IU instead of the recommended dose of 0.5ml of 800IU. The patient developed vitamin D toxicity after consuming a mega dose of vitamin D supplements due to a manufacturer error. Hypervitaminosis D has severe life-threatening consequences like failure to thrive in otherwise healthy-born infants. Regular monitoring of vitamin D supplements administered in infants by medicinal practitioners and strict supervision of all stages of the production process by pharmaceutical companies is crucial to prevent complications from supplement overdose.