OK, this has happened to me twice today. A scripted attachment I've been working on suddenly vanishes from the inventory whilst still being visibly worn. I think this may occur intermittently if a script save fails.
The attachment is still visible and editable but the Drop option is missing from the menu. The attachment does not display in the inventory window and detaching it or logging out results in it vanishing into the ether.
This can result in a loss of time and work if you've been working on the attachment and don't have a back up.
I've reported this in jira, but thought I would warn all creators working on attachments to keep backing things up frequently as this can be quite a nasty one.
Samm
P.S. if you've also encountered this please vote/comment on jira at http://jira.secondlife.com/browse/SVC-1157