The excellent (and honest) Mozart researcher Dennis Pajot of MOZARTFORUM has written an article there that speaks of 3 part fugues based on music by JS Bach which have often been attributed to Mozart. Giving credit where it is due he writes -
In the Köchel Catalogue listed under K404a are six 3-part Fugues arranged for String Trio. To the J.S. Bach (and one W.F.Bach) Fugues are prefixed four anonymous Adagios attributed to Mozart and two slow movements from Bach organ Sonatas:
Nr.1 Anon.Adagio Fugue= WTC I, Nr.8 D# minor (transposed to d-minor).
Nr.2 Anon.Adagio Fugue= WTC II, Nr.12 in F#major (transposed to F)
Nr.3 Anon.Adagio Fugue =WTC II Nr.14 in F# minor (transposed to g minor).
Nr.4 JS Bach Adagio (BWV527) Fugue from Art of Fugue Contrapunctus 8
Nr.5 JS Bach Largo and Fugue from Organ Sonata BWV 526
Nr.6 Anon.Adagio Fugue=WF Bach Fugue in f-minor (Falk No.31/8)
Mozart's authorship of these Fugue arrangements and of the 4 anonymous Adagios was first suggested HYPOTHETICALLY by Wilhelm Rust in 1860, based on Mozart's letters about the "Bach-a-thons" at van Swieten's and the precedent of K405. It must be remembered these arrangements are all anonymous, copied in the 19th Century and do not even mention Mozart's name. Also, K405 was arranged by Mozart for String Quartet and DID NOT have Adagio preludes.
In 1903 Ernst Lewicki attempted to describe the special character of these pieces and remarked it would be advantageous to publish the pieces even though Mozart's authorship could not be proven. Alfred Einstein in 1936 followed with his special line of reasoning to attribute these pieces to Mozart. To Einstein the only other composer besides Mozart who could have written these pieces was Johann Georg Albrechtsberger. However "As able and estimable Albrechtsberger was, a glance at the prelude (quoted in the article) is sufficient to show that no other master than Mozart could have written it". Einstein placed the arrangements in K3 as K404a. The pieces were published by Johann Nepomuk David in 1938 as Mozart's work.
Those interested in the full article can read it on Mozart Forum.
And so, once again, works attributed to Mozart are, at the very least, of dubious Mozartean origin.