Chapter XV of The Innocents Abroad has several interesting points including his interpretation of the story of Abelard and Heloise. He visited two burial grounds, Pere la Chaise and St. Denis. Apparently he found Charlemagne at St. Denis. I looked up Charlemagne in Wikipedia and it has him interred in a vault in Aachen Cathedral, in modern day Germany. This chapter of Innocents is not represented in any of the Alta letters so must be a product of recollection while writing the book. If this is indeed an error on Twain's part, how could this have happened?