"My wife, an experienced nurse, recently switched jobs. The change had been a long time coming. Jana was excited to join up with two doctors whom she had worked alongside previously. She was back with "family"; she had come home.

Her first evening at the clinic a young mother came with her 18-month-old son. He needed his final shot for a routine immunization; his mother came for a physical. Both patients were new to the clinic.

Jana gave the boy his shot, and his mother took him back to the waiting room, where his sister and grandmother sat. The mother then went back to the room for her physical. When Jana went to record the vaccination on the boy's chart, she noticed that the seal on the vial inside her lab coat was unbroken. Quickly Jana realized that she had given the boy the wrong vaccine. She had given him a shot from a different vial—a routine vaccination for children, but the boy had already completed that series of shots months earlier.

Jana told me she gasped when she realized ...