On a fresh WP 3.5.1 installation, after installing and activating the latest version of this plugin, I get the following error in the Dashboard:
Warning: array_key_exists() expects parameter 2 to be array, boolean given in [snipped]/plugins/addthis/includes/addthis_addjs.php on line 53
This only happens once, then it never appears again. I've replicated this behavior multiple times by testing out fresh installations of WordPress.
I assume the problem here is that the code is trying to retrieve and use options that don't exist in the database yet. I would think the options are set the first time the plugin code is loaded, which explains why the warning appears only once.