Borrowing ideas from Literature (specifically I have Jorge Luis Borges in mind who has written "reviews" of and discussed non-existing books, without revealing that they were non-existent of course, as a sui-generis art form), a really interesting fraud would be to back mathematical claims citing non-existing papers (in say, intermediate parts of a proof).
Coming up with convincing such citations, in terms of the claim made but also regarding the journal/cited author chosen, and ideally not easy to find/verify, would be no easy task and the artist, excuse me, fraudulent scholar, would have to spend a visible amount of time and intellectual energy to the task... proving first and foremost that he is a hustler at heart, since he could spend said resources in actually proving something.
In the age of internet and digitized archives, I guess this has become harder to achieve...
I am not claiming originality of this fraud-idea, I just don't know if it has been spotted already in the mathematical (or other) scientific literature.