This case report describes a pediatric cardiac surgery in which electrocardiographic (ECG) changes appeared immediately after the initiation of cardiopulmonary bypass (CPB), followed by severe bradycardia and ultimately flaccid cardiac arrest within minutes. Lab analysis revealed severe hyperkalemia (13.1 mEq/L) and metabolic acidosis, strongly suggesting contamination with cardioplegia solution in the CPB circuit system. CPB was continued, and the heartbeat resumed after potassium removal via hemodialysis, allowing the surgery to proceed as planned. Despite the safe design of CPB and cardioplegia circuits, contamination of the CPB circuit with cardioplegia solution remains a potential cause of unexpected cardiac arrest following CPB initiation.