There is a possibility for race condition in file.php

[zerkms]

        // If file does not exist
        if ( ! $file->isFile())
        {
            // Return default value
            return $default;
        }
        else
        {
            // Open the file and extract the json
            $json = $file->openFile()->current();

            // Decode the json into PHP object
            $data = json_decode($json);

            // Test the expiry
            if ($data->expiry < time())
            {
                // Delete the file
                $this->_delete_file($file, NULL, TRUE);

                // Return default value
                return $default;
            }
            else
            {
                return ($data->type === 'string') ? $data->payload : unserialize($data->payload);
            }
        }

There is a chance to read the file contents while another thread is writing to a file.

I've caught that just once for 2 years, and this is how the exception looked like:

 Kohana_Cache_Exception [ 0 ]: Kohana_Cache_File::get failed to unserialize cached object with message : Trying to get property of non-object ~ MODPATH/cache/classes/kohana/cache/file.php [ 170 ]

So I think that the file was written only partially and $json contained malformed json, what leaded to a NULL in $data.

Proposed solution - add if (!$data) return $default; after decoding