HTTP-error 500 when activated

I get a HTTP-error 500 (Internal Server Error) when I activate the plugin.
Commenting out this fn removes the error, but I cant figure out what’s the problem.

public static function getInstance()
    {
        if (!self::$instance) {
            self::$instance = new self();
        }
        return self::$instance;
    }

PHP version 5.3.31. Plugin 2.1.1, WP 3.5.1

http://ww.wp.xz.cn/extend/plugins/post-snippets/