The Cello Lesson was a painting entered by Henry Ossawa Tanner in the Paris Salon in 1902. The painting disappeared and was assumed lost, but modern scholarship had found that Tanner painted over it with another painting after harsh criticism of his technique and the painting's contents.
It featured people who were close to him in his life and illustrated a private (possibly fantastic) family setting, and he may have been experimenting with the Spanish elongated style when painting it. He had used the style elsewhere (Return of the Holy Women) and may have been nervous about critical reaction to the style and he delayed submitting Return of the Holy Women to the Salon (made 1904, submitted 1906)
A critic pointed out that his wife's arm was badly drawn, that the cello was badly colored and the green pillow cushion probably contained arsenic: was the artist so "misfortunate" as to have seen these? Evidently the criticism about this family painting was strong enough to change his feelings and he covered it up.
It wasn't the only painting that he covered over. For unknown reasons he covered over Judas with The Two Disciples at the Tomb. Christ in the Home of Mary and Martha may also be painted over another painting, as it contains a pentimento thought to be Judas.