i think it could get to zero if you combined a production penalty dungeon square with a worker with a production penalty trait, like "lazy."
Didnt think about that.... but not sure if that is the issue. Ill check, but I am highly ADHD so i dont know if ill remember to come back and post the result.. lol Thx for the heads up regardless. But, just saying, even IF that is indeed the issue, I feel the Mine level should negate at least SOME of that debuff... O_o;