After upgrading wordpress to 3.2 I got this error. Anyone who tried to access the blog got this error. I had to deactivate the plugin in order to have the site up again.
Warning: require_once(/...path.../wp-includes/class-json.php) [function.require-once]: failed to open stream: No such file or directory in /...path.../wp-content/plugins/wp-to-twitter/functions.php on line 52
Fatal error: require_once() [function.require]: Failed opening required '/...path.../wp-includes/class-json.php' (include_path='.:/usr/local/lib/php:/usr/local/php5/lib/pear') in /...path.../wp-content/plugins/wp-to-twitter/functions.php on line 52