Had an issue with the mermaids in Adelaide many years ago while flying a Shrike. While I was in the terminal checking in passengers the mermaids decided to help themselves to the plane and my flight bag. I escort my passengers out to the plane and as the first one ducks under the wing to enter the back door out scrambles a mermaid. He pushes out past the passenger who then turns to re-enter the aircraft and smacks his head on the trailing edge of the flap giving himself a major cut just above the eye. So back we all go into the terminal so I can find some medical treatment for the injured passenger. Left about 40 minutes late.