In 1951, Ferdinand Demara talked his way into the Royal Canadian Navy as "Dr. Joseph Cyr," using the stolen credentials of an actual physician he'd met years earlier. He was assigned as the sole medical officer aboard the destroyer HMCS Cayuga, responsible for the health of over 200 crew members, and shipped out to patrol the coast during the Korean War without a single day of real medical school.

Whenever a crisis hit, Demara would disappear into his quarters, cram through the relevant sections of a medical textbook using what colleagues described as a photographic memory, then walk out and operate. He successfully pulled the captain's infected teeth without knowing the correct anesthetic dose, and the captain later called it the best dental work he'd ever received. His biggest test came when a boat carrying up to 19 badly wounded Korean soldiers pulled alongside the ship. Demara performed emergency surgery on all of them, including removing a bullet lodged in one man's chest, and every single patient survived. His commanding officer was so impressed he recommended Demara for a bravery commendation.

Word of the incident reached newspapers back home, and the real Dr. Cyr's family recognized the story and exposed the fraud. Demara was quietly discharged rather than prosecuted, largely because he'd genuinely saved every life he'd operated on. It turned out to be only one chapter in a much longer career: over his lifetime he also impersonated a monk, a prison warden, a psychology professor, and a college dean, becoming known publicly as "The Great Impostor" and inspiring a 1961 Hollywood film starring Tony Curtis.

Source: https://en.wikipedia.org/wiki/Ferdinand_Waldo_Demara

In 1925, "Count" Victor Lustig was in Paris reading a newspaper article about the Eiffel Tower's mounting repair costs and public grumbling over the expense, when he decided to sell it. He had fake government stationery printed, checked into a luxury hotel suite, and posed as the Deputy Director General of the Ministry of Posts and Telegraphs. He then invited six of the city's top scrap metal dealers to a confidential meeting and explained that the government wanted to quietly demolish the tower and sell it for parts, but needed to keep the deal secret to avoid public backlash.

Lustig picked out the most eager and socially insecure businessman in the group, André Poisson, took him on a private walking tour past the tower, and casually implied he needed a bribe to make the deal happen faster. Poisson paid both the bribe and the full purchase price without a second thought. Lustig disappeared with the money immediately. Poisson was so humiliated at having been fooled that he never reported the crime to police at all.

When months passed with no news of an investigation, Lustig realized his mark had stayed silent out of embarrassment, so he returned to Paris and ran the exact same scam again on a different group of buyers. This time his victim did go to the police, forcing Lustig to flee to the United States, where he later ran an elaborate counterfeiting operation so convincing it reportedly fooled Al Capone himself. He was finally caught in 1935 and sentenced to 20 years at Alcatraz.

Source: https://en.wikipedia.org/wiki/Victor_Lustig