So you're busy working only to notice that one of the emails in your OS X Mail App seems to be stuck! You cannot delete or move the email no matter what you try. When doing so you get a message like this.
The message "LOL Cats" could not be moved to the mailbox "Trash - firstname.lastname@example.org".
No worries there's an easy fix. Simply select the mailbox in question, from the Mailbox menu select Rebuild. That should fix it.