Using latest version of W3TC, this plugin gives me a fatal error when I try to activate it:
Fatal error: Call to undefined method W3_Config::instance() in /.../wp-content/plugins/shopp-cache-helper/shopp-cache-helper.php on line 244
Line 244 says:
$config = W3_Config::instance();
Change that to:
$config = new W3_Config;
And now the plugin can be activated again (and works too).