Warning: call_user_func_array() expects parameter 1 to be a valid callback

I don’t know how it happens and what causes this error, but I have it:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘custom_override_checkout_fields’ not found or invalid function name in /home/…/wp-includes/plugin.php on line 235

and line 235 looks like this

foreach ( (array) current($wp_filter[$tag]) as $the_ )
  if ( !is_null($the_['function']) ){
    $args[1] = $value;
    $value = call_user_func_array($the_['function'], array_slice($args, 1, (int) $the_['accepted_args']));
	}

I was browsing and every solution indicates that something is wrong with theme or plugin, but any solution didn’t help me how to figure out what is the problem.

Can somebody help me please?