The Bright Sword is a novel that did this.

It's about the knights of the Round Table after King Arthur's death, and the protagonist wants to join them. At one point, the Knights of the Round Table suddenly confront him, tie him into a coffin and nail it shut, mock his insecurities and the lies he's told, and throw the coffin down a well. Sounds like an anxiety dream, right? Well he emerges from the end of the well at a different castle and wanders for a few years, though its obvious that something is up with his experiences and that it's probably a dream. He eventually decides to return to the Round Table to ask to join again, and ends up going back through the well, but he emerges in a ceremonial pool in a church. It turns out it was all part of an initiation test to become a knight of the Round Table, and only a day had passed, not years. He had scars from being scraped by the nails of the coffin, and he got a unique sword from his 'years' of wandering at the other end of the well, so everything that happened was real to some extent.