When someone uploads a file which is greater than the max upload limit set in php.ini, the user is shown a message saying "A required parameter(sesskey) was missing" instead of the actual factor.
I noticed this error, while trying to restore a backup.
The code responsible for the correct message is (repository_ajax.php)
Seems it is expecting $_GET['action'] to be set, which is not in this case.
I have not tested in other places to be sure if the problem is localized to the restore page or not.