I just installed and activated PB Responsive Images in WP 3.5.1 with the TwentyTwelve theme. The top of the wp-admin screen displays the following error message:
Notice: wp_register_script was called incorrectly. Scripts and styles should not be registered or enqueued until the wp_enqueue_scripts, admin_enqueue_scripts, or init hooks. Please see Debugging in WordPress for more information. (This message was added in version 3.3.) in /path-to-domain/wp-includes/functions.php on line 2959
In addition, on the plugin's setting page, there is an error message under each format row:
Notice: Undefined property: stdClass::$fallback in /path-to-domain/wp-content/plugins/pb-responsive-images/config-page.php on line 125
Clicking the "restore defaults" or "save changes" buttons results in a "Cannot modify headers information - headers already sent..." warning.