There's also something to rectify both those situations. Go to the memory category that the entry is in and append &multidelete=1 to the URL. You'll then get checkboxes next to all the memories in that category. Check off the ones you'd like to delete, and then click the submit button at the bottom of the page. And there you are! All the memories you checked off are deleted!
(Yes, I know that this is covered in FAQ47 [I put it in there, even!] but since that FAQ is quite big, and since no one really drew much attention to it when it was added, I thought I'd post about it here. :)