I can't remember the artist but there was a story about a forger who snuck in his copies of another artist's paintings. They hung in galleries for years until he spilled the beans. It's now said those fakes bring in more on the market than the originals.
Spot the difference How forgery turned £5,000 painting into £700,000 work of art
The original: the painting by Marinus Koekkoek, where three figures are shown on the path, another heading for the bridge, and a lamb is standing by the stream.
The forgery: in the painting put up for auction as the work of Ivan Shishkin, the figures and the lamb have gone, and a signature has appeared where before there was none.