A question I was asked the other day, which I didn't know the answer to was :
When Enterprise Vault processes moved items (provided it's turned on in the archiving policy) after how many attempts to move the item will it be poison pilled (and no more attempts will be made to move it)?
It is a great question, and one which the person asking the question eventually answered themselves! The answer is that the attempt is made 3 times, before the item is marked to prevent future movements.
2436 10:55:25.040  (ArchiveTask) <8180> EV:M CArchivingAgent::ProcessShortcutItem - '<Message Subject>' this is a moved item that has been poison pilled so do not try and update its metadata again
If you are trying to troubleshoot why items don't appear to have their location updated, look out for the above when you do a DTRACE. You may also want to use the referenced technote below for further information: