authcache with memcache serves old content.

It appears that authcache will use memcache during it's page_cache_fastpath() function. However this occurs during early in the bootstrap DRUPAL_BOOTSTRAP_EARLY_PAGE_CACHE before the variables are loaded. However memcache uses variables in its routines to determine if the content is still valid.

// Determine when the current table was last flushed.
$cache_flush = variable_get("cache_flush_$table", 0);

This means when you edit a page, memcache will invalidate the page cache by setting the cache_flush_page_cache variable.
But when you're loading a page authcache will use the memcache and memcache will serve old content.
// Determine when the current table was last flushed.
$cache_flush = variable_get("cache_flush_$table", 0);

This means when you edit a page, memcache will invalidate the page cache by setting the cache_flush_page_cache variable.
But when you're loading a page authcache will ask memcache for the content, it will check the cache_flush_page_cache which is unset and so return old content.