The Diary of Jack the Ripper
In 1888, a serial killer brutally murdered five London women, and while the perpetrator was never identified or caught, the public came to know him as Jack the Ripper. Over a hundred years later, a Liverpool scrap metal merchant named Michael Barrett produced a journal from Liverpool cotton merchant James Maybrick who'd died in 1889, allegedly at the hands of his wife. The journal contained explicit details of the murders of five women and the passage, "I give my name that all know of me, so history do tell, what love can do to a gentle man born. Yours truly, Jack the Ripper."
There had of course been speculation about the true identity of the notorious murderer for a century, but Maybrick had never been on the list. When Barrett presented the journal, many historians believed it certainly had potential to be genuine. But then Barrett confessed to forging it...and then he retracted the confession. The story got even more confusing when Barrett's estranged wife asserted that her family had been in possession of the journal for decades. Weirdly enough, it's still unknown whether the Maybrick diaries are legitimate and/or whether Maybrick was Jack the Ripper. Experts now believe the majority of the crime descriptions were lifted from press reports and contain many of the details that were publicized and later revealed to be inaccurate.