From the artist's life
Madame X caused such scandal in 1884 Paris — over a slipped strap — that Sargent repainted it and left for London; he later called it his best work.
Source: Wikipedia / WikimediaRealism
John Singer Sargent (1856–1925) was the most dazzling portraitist of the Gilded Age, capturing high society with bravura brushwork and effortless elegance. His scandalous Madame X is now a celebrated masterpiece.
Sargent defined the look of an era; his fluid, confident brush remains a touchstone for portrait painters.
Madame X caused such scandal in 1884 Paris — over a slipped strap — that Sargent repainted it and left for London; he later called it his best work.
Source: Wikipedia / Wikimedia

Key works
Works with a dedicated ArtAtlas page are highlighted in gold.
1884
When the painting debuted at the 1884 Paris Salon, the original version showed one shoulder strap of Gautreau's gown fallen off her shoulder — Parisian society went absolutely feral about it, calling it obscene, and Sargent quietly repainted the fallen strap back onto her shoulder, hoping the scandal would die down.