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?