Hello.
I've tried to reproduce this and I can't seem to cause it to cause this error on my test project. In all cases, I tested recipes which only used 'items' as ingredients, no weapons, or armors. I tried where the first ingredient in the recipe was unconsumable, and also where the second was unconsumable. I also tried with the independent items patch and ItemCore turned on and off. In all cases, the expected items remain in the inventory, and the other items are taken as normal, with no errors thrown or crashes.
Any more insight you can provide to help me track this down more?