Sir Arthur Conan Doyle was an interesting guy. On the one hand, he was the celebrated creator of the highly logical detective Sherlock Holmes. On the other hand, he believed wholeheartedly in fairies. And on the third hand, he successfully used his fictional problem solving skills on actual crime, including the murder of Marion Gilchrist, a wealthy, elderly woman. A petty criminal named Oscar Slater was caught hocking a diamond broach nearby right after her death, and sentenced to life in prison. Doyle suspected that the case was a little too easy, though, and tried to help. He failed. But seven years later, the widow of one of the police officers involved in the case reached out to Doyle with proof that evidence had been withheld in order to protect some powerful suspects among Gilchrist’s family. Doyle was able to use his celebrity to reopen the case and get Slater freed from his sentence (although the real murderer was never caught).

Doyle also once solved an animal abuse case so well that he changed the British justice system. A man named George Edalji was convicted of attacking and injuring horses and cattle, as well as writing threatening letters to a human woman. He lost his license to practice law and served three years hard labor as a result. The evidence was pretty flimsy, though: another horse was attacked while Edalji was in jail. Plus, Edalji had poor vision, and the animals were being injured in the dead of night. Doyle suspected that the police were prejudiced against Edalji due to his Indian heritage, and decided to step in. Doyle's intervention turned the case into a national news story. This annoyed the police so much that the chief constable wrote Doyle an anonymous letter telling him to back off or lose his "kidneys and liver."

Doyle wouldn't back off, though. He kept publishing his opinions and stoking public anger, until Edalji's case was reopened and his name finally cleared. He won the right to practice law again. The case was considered such a debacle by the British government, though, that they founded the Court of Criminal Appeal in 1907 to avoid future miscarriages of justice.

Famous mystery writer Agatha Christie famously went missing herself in 1926. She disappeared for a total of 11 days, and her fellow mystery writers decided to take on the case. Doyle gave one of Christie's gloves to a medium, hoping to find her by magic. This failed. Dorothy L. Sayers, author of the Lord Peter Wimsey mysteries, took a more sensible approach and investigated Christie's house and car. Sayers didn't actually find all that much either, although she did use the experience as inspiration for her novel Unnatural Death.

In the end, the true sleuth was the hotel staff member who recognized Christie hanging out at a spa. A confused Christie was collected by her husband, with seemingly no memory of where she had been for the last 11 days.

Poor Agatha Christie. As a former chemist's assistant, she knew a heck of a lot about poisons, and she used her expertise for the murders in her books. But she never intended to inspire (and then help solve) a real poisoning. Her novel The Pale Horse contained quite a few details about the little-known metal thallium. Real life murderer Graham Young started experimenting with thallium the same year her book was published, eventually using the poison to murder three people. Thallium was such a rare choice of chemical weapon that Christie's book ended up a part of his trial, and Christie herself expressed concern that she'd given Young a murderous instruction manual. But a few years later, a nurse just so happened to be reading The Pale Horse, and used the book to figure out the mysterious symptoms of one of her patients. It was thallium. The imprisoned Young was even consulted as an expert on thallium poisoning, and the patient survived thanks to Christie's writing and Young's horrific experiments.

Edgar Allan Poe's short story, "The Murders in the Rue Morgue," is often considered to be the first piece of modern detective fiction. Establishing the detective genre doesn't make one a detective, though. This was glaringly true in the case of Mary Rogers, a beautiful young woman and a local celebrity to New York’s literary set. She worked in a cigar shop that was frequented by great writers of the day. That is, until the day her body was discovered bobbing in the river. The mysterious death of Mary Rogers was a media sensation, and Poe decided that he would be the one to solve it. Or rather, he wrote a short story based on Mary Rogers' murder (because no one writes a tragic dead woman like Poe), and then claimed that his story revealed the truth of the actual case in his introduction: "All argument founded upon the fiction is applicable to the truth: and the investigation of the truth was the object."

In reality, though, Poe had no idea who killed Mary Rogers, and his story makes some vague hints at an evil naval officer and a botched abortion without offering any evidence. A later court case even floated the idea that Poe had been paid off by Mary's murderer to write a sensational story about her death and shift focus away from him. The murder remains unsolved to this day.