Bug displaying undo arrov for registered users.

You need change line 245 to

$criteria['uid'] = $criteria['vote_source'] = $user->uid;

and line 191

$criteria['uid'] = $criteria['vote_source'] = $user->uid;

Otherwise for registered users $criteria['vote_source'] will be null, and they cant undo their votes.
Also will be better to allow other modules possibilities to change entire criteria:

drupal_alter('plus1_source', $criteria);

instead:

drupal_alter('plus1_source', $criteria['vote_source']);