Reliability of technical systems in the XX. and XXI. century, due to the explosive development magnitude improved and they are still improving. These things we live every day of everyday life, because objects of every day of our lives are becoming increasingly complex belongings, but in spite of increasing reliability. In contrast, human reliability, almost nothing has changed, is not a quantum leap in the manifestations of human error and lot of terms. In a well-planned operation process the weak link is the man, but as the systems reliability is not 100%, so it is necessary to have some level of human intervention. The article aims to examine the impact of human intervention in the systems reliability through an example.