This one would be interesting. Ferdinand Waldo Demara was supposedly one of, if not the greatest, impersonator who ever lived. Purportedly, he changed identities regularly and became extremely competent in the fields he was pretending to be trained in. He even went so far as to successfully perform surgery on multiple patients with no previous training, save a surgery textbook he would glanced at before operating; or so the story goes. The wiki link is listed below:
http://en.wikipedia.org/wiki/Ferdinand_Waldo_Demara