Thousands of warnings about splitting object, can't turn off

Thanks for the response. Currently in our local_settings.php (our settings.php is default except for the include of local_settings.php) we have the following:

// Memcache settings
$conf['cache_backends'][] = 'sites/all/modules/contrib/memcache/memcache.inc';
$conf['cache_default_class'] = 'MemCacheDrupal';
$conf['cache_class_cache_form'] = 'DrupalDatabaseCache';

// List all the memcache servers, and then ensure the current one is first.
// Taken from https://groups.drupal.org/node/191998#comment-633648
$memcache_servers = array(
  '10.106.176.11:11211' => 'default',
  '10.106.176.12:11211' => 'default',
);

$conf['memcache_servers'] = array();
// Connect to the local memcache server first.
$key = $_SERVER['SERVER_ADDR'] . ':11211';
if (isset($memcache_servers[$key])) {
  $conf['memcache_servers'] += array(
    $key => $memcache_servers[$key],
  );
}
// Add the rest of the servers into the mix.
$conf['memcache_servers'] += $memcache_servers;
$conf['memcache_log_data_pieces'] = 10;

$conf['lock_inc'] = 'sites/all/modules/contrib/memcache/memcache-lock.inc';

As I mentioned above I have added a flag to see if it even gets to this stage

if (variable_get('memcache_log_data_pieces', 2) && $piece_count >= variable_get('memcache_log_data_pieces', 2)) {
//Check if conditional is hit.    
watchdog('memcache_split_test', variable_get('memcache_log_data_pieces', 2) . ' - ' . $piece_count);
    if (function_exists('format_size')) {
      $data_size = format_size(strlen($data));
    }
    else {
      $data_size = number_format(strlen($data)) . ' byte';
    }
    register_shutdown_function('watchdog', 'memcache', 'Spent !time ms splitting !bytes object into !pieces pieces, cid = !key', array('!time' => timer_read('memcache_split_data'), '!bytes' => $data_size, '!pieces' => $piece_count, '!key' => dmemcache_key($key, $bin)), WATCHDOG_WARNING);
  }

And that watchdog message never gets created, which suggests that this conditional is false - as I would expect (but it obviously is TRUE, because the shutdown function is registered). However, I have tried as you suggested, changing the default value to 10, and that works. But I am reluctant to patch a contrib module for something that should work with the config I have. I can't think where the variable is being overwritten, and both drush php-eval and drush vget return the correct conf setting

When you say there might be things that shouldn't be written to cache_bootstrap-variables, can you point me in the direction of something that I can learn how to check and configure that? Because it seems googling is particularly bad, or there's very little information out there for me to tackle the issue - clearly I'm searching for the wrong thing. I would much rather fix the source than just turn off the logs!

I did consider that possibility, but I always thought watchdog was available from bootstrap.

I tried putting it at the bottom of settings.php, and no joy. The memcache module is picking up the memcache server details fine, and all the other memcache conf settings, so I can't figure out why this is not picking up this one variable.

I've been restarting apache and php-fpm on every code push. I was thinking that it is possible that the domain access include was interfering with this, but every related issue I find has shown that placing domain access last, AFTER everything else is the solution to any issues. Either way I'm confused I can't see why it would still pick up all the other memcache conf variables in my local_settings include, and have the correct value when outputting to drush vget. I'll have to keep looking into exactly what domain access is doing with the conf array.

Also, can you elaborate on this please?

Note, it's also a good idea to try and keep the size of your cache tables smaller if possible. Perhaps there's things currently being written to cache_bootstrap-variables that don't need to be there?

I'm still getting this error, I've gone through and completely disabled domain access and it still ignores/overwrites the variable, and only this variable.

I've tried tweaking the cache bins, which took me a while to find anything useful with regards to. Either way, it hasn't made a difference, there is something very odd going on with this memcache install.
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The mere existence of this module causes errors on other servers, despite it not being enabled. I get a lot of these:

WD memcache: You must enable the PHP memcache (recommended) or memcached extension to use        [error]
memcache.inc.
WD memcache: Failed to connect to memcache server: 127.0.0.1:11211

In drush. The registry has been rebuilt, nothing in the settings pointing to memcache.inc, nothing in 'cache_backends', and caches cleared over and over.