Mona Lisa
Leonardo da Vinci is thought by some to have begun painting the "Mona Lisa" in 1503 or 1504 in Florence, Italy. Although the Louvre states that it was "doubtless painted between 1503 and 1506", the art historian Martin Kemp says there are some difficulties in confirming the actual dates with certainty. In addition, many Leonardo experts, such as Carlo Pedretti and Alessandro Vezzosi, are of the opinion that the painting is characteristic of Leonardo's style in the final years of his life, post-1513. Other academics argue that, given the historical documentation, Leonardo would have painted the work from 1513. According to Leonardo's contemporary, Giorgio Vasari, "after he had lingered over it four years, [he] left it unfinished". Leonardo, later in his life, is said to have regretted "never having completed a single work".
Circa 1504, Raphael executed a pen and ink sketch, today in the Louvre museum, in which the subject is flanked by large columns. Experts universally agree it is based on Leonardo's portrait of "Mona Lisa". Other later copies of the "Mona Lisa", such as those in the National Museum of Art, Architecture and Design in Oslo and The Walters Art Museum in Baltimore, also display large flanking columns. As a result, it was originally thought that the "Mona Lisa" in the Louvre had side columns and had been cut. However, as early as 1993, Zöllner observed that the painting surface had never been trimmed. This was confirmed through a series of tests conducted in 2004. In view of this, Vincent Delieuvin, curator of 16th-century Italian painting at the Louvre museum states that the sketch and these other copies must have been inspired by another version, while Frank Zöllner states that the sketch brings up the possibility that Leonardo executed another work on the subject of Mona Lisa.
Yves Chaudron
Decker asserts that six Chaudron copies had already been sent to the United States ahead of the theft and while the stolen original remained in France, Valfierno followed his fakes and sold each for up to 300,000. The original remained hidden for two years until Peruggia, presenting himself as Leonardo Vincenzo, tried to sell it in Florence, Italy. A museum administrator there was suspicious, reported the attempted sale and Peruggia was arrested. The original "Mona Lisa" was returned to the Louvre in 1913. 
More than 100 years later, none of these six paintings has ever surfaced, bringing into question if they ever in fact existed. Further, the only evidence that the "famed forger" Yves Chaudron even existed is Karl Decker's 1932 "Saturday Evening Post" article. There are no known photos of the man or relatives or any biographical details outside of this incident. None of his other forged works has ever come to light, and it is quite impossible to be a 'famed forger' if none of your forgeries is discovered. Decker's entire 1932 article may be a work of fiction that, much like forgeries in the art world, has fooled many who have read it.