Nothing strikes fear into the heart of an art collector or museum like the possibility that a prized work of art might actually be a worthless forgery. And yet, despite the best efforts of experts to safeguard against such chicanery, fake works masquerading as priceless original continue to litter the market.

Art history is rife with high profile scams, such as that of Han van Meegeren, a disillusioned painter who developed a complicated system of baking his Old Master-style work to age it, and subsequently sold $60 million in fake Vermeers to world class museums and Nazi leader Hermann Göring during the late 1930s and early 1940s. (While current art historians are quick to slam his work as obviously inferior, Van Meegeren actually had to paint a “new” Vermeer at trial to prove he had not sold the Nazis a priceless original.)

In recent weeks, the art world has been rocked by perhaps the biggest forgery scandal to hit the art world since Van Meegeren’s unmasking. The extent of the Old Master forgery ring is as of yet unknown, but Sotheby’s has already issued a refund to the buyer of a $10 million Frans Hals portrait, sold in 2011 in a private sale through London dealer Mark Weiss. James Martin’s Orion Analytical, a Williamstown, Massachusetts-based company which investigates artworks, found modern-day materials in the canvas, proving it to be a forgery.

Other paintings are now also implicated, including a Lucas Cranach the Elder, from the collection of the Prince of Liechtenstein, that was seized by French authorities from the Caumont Centre d’Art in Aix in March. An Orazio Gentileschi painting on lapis lazuli, also sold by Weiss, and a purported Parmigianino have been identified as suspect as well. Rumor has it that works by up to 25 different Old Master paintings may be involved. (For a break-down on what we know so far, read “The Frans Hals Forgery Scandal, Explained.”)

All the paintings appear to have originated with one man, an obscure French collector-turned-dealer named Giulano Ruffini. The works appear to have had next-to-no provenance, save that they came from the collection of French civil engineer André Borie. Ruffini insists he never suggested they were the real deal, and that eager dealers were the ones to declare his paintings Old Master originals.

The art world was quick to fall in line, with London’s National Gallery displaying the Gentileschi and the Pamigianino popping up at New York’s Metropolitan Museum of Art. At one point, the Louvre in Paris launched a fundraising campaign to buy the Hals, dubbing it a “national treasure” after it was authenticated by France’s Center for Research and Restoration.

The fact that so-called experts were fooled suggests that connoisseurship, long the gold standard of Old Master authentication—which does not rely on science but a less tangible ability to sense the presence of a great artist’s hand—can no longer be relied upon.

Perhaps most frightening of all, there is no telling how many fakes still lie in plain sight, accepted as originals by experts and the public alike. Famed contemporary forgers such as Wolfgang Beltracchi and Mark Landis, for instance, have infiltrated many museum collections. In 2014, Switzerland’s Fine Art Expert Institute estimated that 50 percent of all work on the market is fake—a figure that was quickly second-guessed, but remains troubling.

As artnet News continues to follow this developing story, here are just a few of the most high-profile art forgery cases that have come to light in recent years.

1. Knoedler Forgery Ring
Glafira Rosales, an obscure Long Island art dealer, her boyfriend, and his brother enlisted Pei-Shen Qian, a Chinese artist in Queens, to paint Abstract Expressionist canvases in the style of such masters as Jackson Pollock, Mark Rothko, Robert Motherwell, and others. The venerable Knoedler gallery, which closed in 2011 as the forgeries came to light, still claims they believed Rosales’s story that the works were part of an undocumented collection sold directly by the artists to an anonymous “Mr. X.”

Whether or not the gallery was in on the scheme (one of the many claims against Knoedler went to trial earlier this year but was settled before gallery president Ann Freedman could testify), they still sold the worthless works for $80 million, leaving a slew of lawsuits, some still unresolved, in their wake.

2. An Inside Job in Uzbekistan
Even possession of an original work of art doesn’t protect you from worry. The Uzbek State Art Museum, for instance, discovered that over the course of 15 years, its collection was systematically pillaged by a group of employees, at least three of whom have since been convicted.

Over 25 works by European artists, including Renaissance great Lorenzo di Credi and modern Russian artists Victor Ufimtsev and Alexander Nikolaevich, were swapped out for copies. The employees then sold the originals on the black market for a pittance.

3. Thousands of Fake Bronzes Sold for Millions
A forgery ring busted in 2011 is still having repercussions across the Alberto Giacometti market. Dutch Giacometti forger Robert Driessen made €8 million ($8.9 million) selling forged sculptures, along with thousands of fake bronzes, before his misdeeds were discovered. In 2015, the case again made headlines when a German dealer was caught trying to sell one of the works still at large to an undercover agent.

4. A $250 Million Heist in Turkey
Forgeries again came into play at Turkey’s State Art and Sculpture Museum in Ankara, where a group of museum officials and criminals are believed to have teamed up to steal some 302 works from the institution between 2005 and 2009. The crime was discovered in 2012, when the museum realized that 46 pieces in the collection had been replaced by copies. Another 30 works also raised suspicion.

The case was solved in late 2014 thanks to a tip from an anonymous caller.

5. An Art Dealer Sells Fakes Through eBay
Using a variety of aliases, Michigan-based dealer Eric Ian Hornak Spoutz created fake paperwork to help sell dozens forged works by Willem de Kooning, Joan Mitchell, and other artists. Because the transactions were conducted through eBay, Spoutz was charged with wire fraud.

He may have even taken in the Smithsonian, which has six works in its the collection that originated with Spoutz.

6. A Covert Investigation Blows Open a Spanish Forgery Ring
A Spanish forgery ring dealing in fake Joan Miró, Pablo Picasso, and Henri Matisse drawings was uncovered by Spain’s Civil Guard in January 2015 after a six-month investigation dubbed Operación Mirones (Operation Voyeurs). The authorities first spotted the fakes entering the country in July 2014, but opted to put the suspect under surveillance.

The investigation uncovered a Zaragoza-based art dealer’s scheme to sell the counterfeit works for hundreds of thousands of Euros, and nine fakes were recovered during a raid that put a stop to the illicit operation.

7. Lee Ufan Authenticates Forgeries
In an unusual case, South Korean art star Lee Ufan claimed 13 suspected forgeries as his own original work, even after South Korean art dealer named Hyeon admitted they were counterfeits. Seoul police began investigating the case in February, and indicated that there were as many a 50 fake works at hand when Hyeon was indicted in June.

Nevertheless, Ufan was unswayed by the evidence, insisting that “an artist can recognize his own piece at a glance.”

8. Furniture can be forged too…
In June, antiques dealers Laurent Kraemer, head of Paris’s venerable Kraemer Gallery, and chair specialist Bill Pallot, were arrested on suspicion of selling the Palace of Versailles four counterfeit medallion back chairs for €1.7 million ($1.9 million). Counted as “National Treasures,” the chairs were thought to be among a group of 13 created by Louis Delanois for the Palace living room in 1769, where they belonged to Louis XV’s last mistress, the countess du Barry.

The problem is that there are now more than 12 chairs from the set floating around (the 13th, a larger version made specially for the King, is thought to be lost). The Kraemer Gallery maintained its innocence, but withdrew fromthe Biennale des Antiquaires, which was held in Paris in September.

9. But probably not facial modifications…
French artist Orlan sued American pop star Lady Gaga for forgery after the release of the 2011 hit video “Born This Way,” but ultimately lost her case. Orlan pointed out similarities to her piece Bumpload (1989), in which she added prosthetic ridges to her face, and Woman With Head (1996), which featured a decapitated head on a table. After losing her $31.7 million lawsuit, Orlan was ordered to pay the singer and her record label €20,000 ($22,000)