Fritz Kreisler is famous for writing pastiche pieces for violin and piano, and, according to legend, he picked the names of the composers who "inspired" the styles of those pieces out of a music encyclopedia, even though he didn't have any way of knowing what each composer's style might have been. Back in the first decades of the 20th century nobody really knew what Francois Francoeur's music sounded like, and we still only have a few examples. The well-known E-minor cello sonata could have been written by Konstantin Petrov Popov, who edited a Russian edition from 1907, or Pepov could have been a pseudonym for another cellist/composer who really wanted to have a good baroque-style piece to play (Konstantin Petrov Popov's only listing in the IMSLP is as the editor of the E minor Francoeur Sonata). I don't have any kind of absolute proof, other than a hunch, but I'm not totally convinced that the cello sonata is written by the same person who wrote these violin sonatas sometime between 1720 and 1733.
One of my favorite musical hoax pieces is the one that Samuel Dushkin wrote in 1924. He had the nifty idea to claim that it was written by a contemporary of Mozart's named Maria Theresia von Paradis who was a pianist and a composer. She was known mostly for the high quality of her playing, the pieces that were written for her, and that fact that she was blind. This portrait demonstrates that fact by representing her eyes without irises.
Other people have suggested that Dushkin used the Romanze of the Weber Violin Sonata Opus 10, No. 1 as a model (the second movement of first sonata in this volume), but I think it owes far more of its inspiration to the Sicilienne part of Fritz Kreisler's "Sicilienne and Rigaudon."
Here's the "Paradis" Sicilienne performed (beautifully) by Jaime Laredo:
and here's the Kreisler piece ("in the style of Francoeur") from 1910 that is strikingly similar in spirit and in "violin-ness." It also just happens to have the same title as the Dushkin.
Here's a nifty NPR piece about the subject of "musical fraudsters."