Goethe in Johann Tischbein's portrait and the explanations for his two left legs

Remembering Goethe today, 188 years since his death. I do like the way that, in the portrait by his friend Johann Tischbein, who won his fame precisely through his portraits of Goethe, he has settled himself in the pose of an ancient Roman upon a ruined Egyptian obelisk, beneath an ivory-coloured rug reminiscent of the sleeveless cloak-capes so popular in those years, known as the almaviva and the talma.

It may seem that in this portrait Goethe has two left feet. Art historians have no common opinion as to how this came about. The audio guide at the Frankfurt museum explains it by saying that the portrait was finished by the artist's pupil, and his blunder went unnoticed