Background: Medication errors due to different reasons may alter the outcome of all patients, especially patients with drug poisoning. We introduce one of the most common type of medication error in the present article.Case: A 48 year old woman with suspected organophosphate poisoning was died due to lethal medication error. Unfortunately these types of errors are not rare and had some preventable reasons included lack of suitable and enough training and practicing of medical students and some failures in medical students’ educational curriculum.Conclusion: Hereby some important reasons are discussed because sometimes they are tremendous.We found that most of them are easily preventable. If someone be aware about the method of use, complications, dosage and contraindication of drugs, we can minimize most of these fatal errors.