Boarding Pass Saga

Recently a workmate flying to Wellington boarded an Air NZ plane only to discover someone sitting in his allotted seat. On checking their ticket, my workmate discovered he was on the wrong flight heading to Blenheim. He hurriedly left that plane and reached the correct flight in time for departure. However he discovered a female in his seats and her ticket is a flight to Invercargill. I wonder Air NZ staff is really checking passengers' tickets?