I have lost a set of treasure keys (while trying to transfer from one character to another during PoE) and a fully upgraded Red Death pulse rifle (trying to transfer from one character to another with a full primary inventory. I understand that during these scenarios I was unable to transfer the items but they have completely gone missing and do not show up on any character, leaving me with having to grind again to upgrade and collect enough resources to get them back. This happened twice in a week. Never happened once during the full first year.