In the recently deciphered letters from Mary Queen of Scots, a large number of systematic encryption errors were found and attributed to confusion as a result of concurrently using at least one other cipher key to communicate with a different recipient. In this paper, we further analyze such cross-cipher errors in those letters and identify additional cipher keys involved. This analysis also reveals valuable insights on the secret communications of Mary, Queen of Scots. We employ several techniques including statistical analysis, which may be applied to the analysis of encryption errors in other collections of historical enciphered documents.
