Sometimes under high load the call to Zend_Cache_Frontend_Function::call() returns NULL.

Looking at the source code below seems that it is possible if the cache expires between the calls $this->test() and $this->load().

     /**
     * Main method : call the specified function or get the result from cache
     *
     * @param  string $name             Function name
     * @param  array  $parameters       Function parameters
     * @param  array  $tags             Cache tags
     * @param  int    $specificLifetime If != false, set a specific lifetime for this cache record (null => infinite lifetime)
     * @param  int   $priority         integer between 0 (very low priority) and 10 (maximum priority) used by some particular backends             
     * @return mixed Result
     */
    public function call($name, $parameters = array(), $tags = array(), $specificLifetime = false, $priority = 8)
    {
        $cacheBool1 = $this->_specificOptions['cache_by_default'];
        $cacheBool2 = in_array($name, $this->_specificOptions['cached_functions']);
        $cacheBool3 = in_array($name, $this->_specificOptions['non_cached_functions']);
        $cache = (($cacheBool1 || $cacheBool2) && (!$cacheBool3));
        if (!$cache) {
            // We do not have not cache
            return call_user_func_array($name, $parameters);
        }
        $id = $this->_makeId($name, $parameters);
        if ($this->test($id)) {
            // A cache is available
            $result = $this->load($id);
            $output = $result[0];
            $return = $result[1];
        } else {
            // A cache is not available
            ob_start();
            ob_implicit_flush(false);
            $return = call_user_func_array($name, $parameters);
            $output = ob_get_contents();
            ob_end_clean();
            $data = array($output, $return);
            $this->save($data, $id, $tags, $specificLifetime, $priority);
        }
        echo $output;
        return $return;
    }