In the Emerald Viewer Review I wrote about Inventory Loss. Inventory loss is now also a problem with the standard Second Life viewer (not that it is new thing but the current problem seems to client side as opposed to the ongoing asset server side problems). Since the problem is in the basic SL viewer code it is also likely to show up in most 3rd party viewers.
I also wrote about failed delivery of items purchased (See: Failed Inventory Deliveries in Second Life) This loss thing is not that failed delivery problem, but it could also cause what appears to be a failed delivery.
Linden Lab has a wiki where steps for recovering your lost items are covered. The Second Life Wiki article is: Inventory loss

