How this ghost has been made with a real time iphone photography, to prank people?

Someone took a picture with iPhone and this happened, any one knows any app to do such realistic ghost or any good technique to do that? Apparently there was no time to work on photo and it is very fast job. How this ghost has been made with a real time iphone photography, to prank people

Ok

OK. Guilty as charged. I'll come quietly.

Ghost reflection in glass, subject behind glass. Old movie trick.