Odd I never ran into it during testing, doing most of the same things with production as now. I'll try to get it to break reliably tonight.
Edit:
Fairly sure It's not how it's happened to me during normal play, but I think got it to happen.
spawned some materials and built a tailor, set 2 bills for infinite parkas, each with different materials.
started one person crafting on one bill, then sent em elsewhere
started a second person on the second bill, then sent em elsewhere
reduced count on one of them to 0.
turned it back to infinite.
the unfinished item is now in a stockpile and the only option i get if i right click it with it's creator selected is to try and haul it.
Seems it's bill is set to 'null'
https://drive.google.com/folderview?id=0BzbMtxHC8p0sMGYzVVpiT3Q4N0E&usp=sharing