I've had that happen too, mostly with my blacksmiths and wizards. I make the item, it's in my inventory, I give the item, and I get a note pop up that I delivered an empty box or something. At first I thought it was because I had started fast forwarding the crafting action, but it would happen even when I didn't fast forward through. I do wonder if it has to do with mood/focus, but I've never paid that much attention. I don't normally carry around extra potions/swords with me so when I get those tasks I make them on the spot. I find it happens to me more often when my tasks for the day are two crafting items that I make back to back.