Lol, you have no idea how many times I have done this! I once NPC-ed over 1 million gil worth of sheepskin because I thought it was fleece (I didn't notice until I sat back down to start crafting). But I can live with 1 million gil loss.
What really annoys me is when I accidentally NPC unique items which I can neve get again. I NPCed Onion gear including my helm and also some R30 class quest gear.
My suggestion is to just spam GM-call until they give it back to you or send you to jail.