I've got xemacs built from cvs (latest unstable, just rebuilt as of an hour
ago).  Over the past few months the mercurial.el mode that I was using
stopped loading.  I'm currently getting the following error when I try to
load the mode:

Structure formation error: Invalid prefix keys in sequence, ?\^C, "^Ch"

Doing the obvious thing and changing the \C-ch binding to something else
returns the exact same error, which puzzles me a lot.

The mode can be found at
http://www.selenic.com/hg/raw-file/c0271aba6abe/contrib/mercurial.el