What factors may have existed that caused the surgeon to select the wrong tooth?

An oral surgeon was performing a surgical removal of lower third molar, which was
completely impacted. None of the third molars (on either side) were visible.
According to the clinical record, the right third molar was to be extracted. However, the X-ray on the view box appeared to show that it was the right third lower molar that was impacted and that the left third lower molar was absent. The oral surgeon made the incision, raised the flap and started the osteotomy. The impacted
molar did not appear, so the surgeon enlarged the osteotomy. The surgeon finally realized that the right third molar was not there and that
he had made a mistake when he had reviewed the clinical notes earlier and planned the operation. Furthermore, the dental assistant had displayed the X-ray in the wrong position, reversing the left and right sides of the mouth.
Questions
1.What factors may have existed that caused the surgeon to select the wrong tooth?
2.What may have caused the assistant to put the X-ray in the wrong position?