The attribution of the Toccata and Fugue in D minor to Bach has been challenged since the 1970s by a number of scholars. There are atypical features throughout the piece and even frankly broken chords. For some time there were theories that it was the product of a young and adventurous Bach, or that it was in fact a piece meant as an organ test.. There's the name -- Bach's generation would have called it "Praeludium et fuga," not Toccata and Fugue -- and a progression of notes Bach never would have allowed.
"Bach's greatest inspiration is invariably revealed through his complete mastery of the 'rules,' "
The evidence of rule-breaking includes doubling at the octave and the curious minor cadence that ends the Toccata, both not heard elsewhere in Bach's organ output (usually even a work in a minor key concludes with a major chord). The Toccata also brims with harmony and counterpoint bordering on simplistic for the masterful composer.
"No other Bach fugue contains such feeble part-writing," writes Fox-LeFriche, citing the "complete absence of contrasting rhythm, contrary motion or a least a few notes that don't slavishly follow the subject."
In short, the Toccata and Fugue approaches nothing Bach ever wrote for the organ, or ever wrote at all.
"It is certainly very different than any of his organ works," says Don Fellows, organist at St. Paul Cathedral in Oakland. "There are parts that don't fit the hands."
So if Bach did not write the Toccata and Fugue, then who did?