Found this with 1.6.3, seems the same in current svn.
includes/User.php function matchEditToken calls memsess_key, but function
SetupSession only includes MemcachedSessions.php if $wgSessionsInMemcached is true.
I got a PHP error from this when clicking "Show Changes" during editing a page,
but tried a few other times on other edits and show changes worked ok. So,
don't know how to reliably reproduce, but the code does seem to expect that
function when it might not be included.