I think I found it. Problem (bug): http://core.trac.wordpress.org/ticket/18322
Solution: http://codex.wordpress.org/Function_Reference/stripslashes_deep
$_GET = array_map('stripslashes_deep', $_GET);
$_POST = array_map('stripslashes_deep', $_POST);
$_COOKIE = array_map('stripslashes_deep', $_COOKIE);
$_SERVER = array_map('stripslashes_deep', $_SERVER);
$_REQUEST = array_map('stripslashes_deep', $_REQUEST);
Note: As suggested by @Alexandar O’Mara, you might want to reconsider overwriting the superglobals like this. If it’s appropriate for your situation, for example, you might just “strip locally” using an alternative like $post = array_map('stripslashes_deep', $_POST);
Also see @quickshiftin’s excellent answer.