[Standards] Id risks

Hi all, Further to the note in the LC thread on XEP-0424, I'd really like to have a document (XEP, probably) that answers these questions: What are the risks of choosing an stanza identifier that is not unique? Does this still matter if the stanza identifier is unique to the session? To the sending jid? To the bare jid of the sender? What about missing off the stanza id attribute entirely? What about MUC? PubSub? Etc? If people have opinions, write away, and I'll volunteer to collate these into a XEP (or, possibly, a patch against XEP-0359). My reasoning is that we seem to vaguely know things can get Bad, but I for one can't find where we've documented these Bad Things. I'd be delighted to be corrected, as ever. Dave.