A bunch of unrelated wordpress using same memcached don’t store correctly their datas

If WordPress is using APC it will use it for its object cache, and since APC is per machine, not per cluster, the two will diverge, giving you problems if you’re trying to run 1 site off of two machines.

In that scenario you will want 1 memcached deamon running, and configure your 2 servers to use the 1 instance.

But you mentioned 2 servers with unrelated sites.

So in this case, we have the same issue as here:

http://wordpress.org/support/topic/many-wp-installations-on-the-same-server

The plugin author says:

By default it should. But that requires that you use unique table
prefixes (They all cannot be wp_).

To use it on many sites where the table prefixes are the same, you can
add a constant to wp-config.php called WP_CACHE_KEY_SALT, such as:

define(‘WP_CACHE_KEY_SALT’, md5(DB_NAME . FILE));

md5(DB_NAME . FILE) can be replaced by whatever you want, just as
long as it is unique per site.

So in effect, you’ve already stumbled upon the solution for memcache, and the cause.

So perhaps, if you rename your table prefixes so that they’re unique, memcached will work

To ellaborate, if we look at the memcachedredux plugins key method:

function key( $key, $group ) {  
    if ( empty( $group ) )
        $group = 'default';

    if ( false !== array_search( $group, $this->global_groups ) )
        $prefix = $this->global_prefix;
    else
        $prefix = $this->blog_prefix;

    return preg_replace( '/\s+/', '', "$prefix$group:$key" );
}

and later in the constructor:

    global $blog_id, $table_prefix;
    $this->global_prefix = '';
    $this->blog_prefix = '';
    if ( function_exists( 'is_multisite' ) ) {
        $this->global_prefix = ( is_multisite() || defined( 'CUSTOM_USER_TABLE' ) && defined( 'CUSTOM_USER_META_TABLE' ) ) ? '' : $table_prefix;
        $this->blog_prefix = ( is_multisite() ? $blog_id : $table_prefix ) . ':';
    }

Here we see that either the prefix is ” or it’s the blog ID or the table prefix.

As such there is no handling of multiple sites -> 1 memcached. So change your table prefixes, test if it works. If it does not, then you will need to modify the plugin