I've read The Once and Future King and Mists of Avalon, watched Excalibur countless times, read and watched the Tristan and Isolde tale in many forms (a story that predated and may have influenced the Arthurian tales). I've been to places in England where historical evidence of the stories can still be seen. I've even written a retelling of one of the Arthurian Romance Tales - The Lady of the Fountain - as The Deaths of All We Are in the most recent issue of Heroic Fantasy Quarterly. So how did I not know that a real Sword in the Stone existed? According to the article, though, this sword found its way into the stone around 1180CE, while tales of King Arthur (or at least a famous military commander - Artur, Arturo, etc) began in oral tales probably centuries before a 9th century monk recorded written references in his Historia Brittonum, and they referred to a man who lived around the late 5th and early 6th century CE. It seems possible that the stories influenced the knight's action (or, cynic that I am, that building a chapel around the sword came first and the story second considering the booming business of relics in the middle ages), and not the other way around, but still, cool beans to learn about this.
Sword in the Stone at the Monte Siepi Chapel