Embrace thyself?

I find it interesting when authors go with the embrace thyself theory. In a few of the books I've read the author has decided that the character will manage to change themselves completely by accident. This is always interesting since it's presumably hard to do. Usually you need a large amount of blood exchanged. So it has to be a pretty gruesome scene to have had enough blood exchanged to do it. Not many people are up for that kind of carnage.