Rahul discusses the many ways in which people become authors of academic papers they didn't contribute to: Giving "credit" where it's not due. After covering fraud as well as "courtesy" (unearned authorship given to "senior figures ... merely in recognition of their position or funding"), he gets to some very interesting routes to authorship. Here's one:
I know of two older papers where one author, famously, was not a contributor to the paper. The first is this one, regarded as a classic; the authors were George Gamow and his student Ralph Alpher, and Gamow included Hans Bethe, who had no connection with this work, purely so that the author list would read "Alpher, Bethe, Gamow." (If that joke is Greek to you, never mind.) I can't remember whether Bethe was "in" on it, but he did not protest, at least not publicly.