Lost items directly from inventory :(

I had the one of a kind Relay for Life Vanity Set by Sabrina Doolittle, but I looked for it in inventory today and can't find it. I lost it directly from inventory.

Does this happen frequently in SL? I have things I really don't want to lose. Things that I can't replace. These items are saved on server side? Any suggestions for me on recovering these items other than asking the creator for another copy?

I'm working on very large unique project. What can I do to safeguard it from being lost?

TIA

Update: I found the lost items. I cleared the cache from the network tab in preferences. Thanks Amber Linden!