I hope this isn't a repeat, I did search for something similar, but didn't come up with anything. I came across a bug where I had a lumber mill on a plot before I discovered coal. Once coal was discovered I had 3 plots of coal. I had created mines on two of them, but the lumber mill was already on top of the third. Once mousing over each mine it stated I had a production of +3 coal per turn (for each) for a total of +6 total production. However, when mousing over the lumber mill, it also had a +3 production per turn. I did look at my resources and it only showed a +6 production. So technically the lumber mill showing coal production is incorrect, not the total production amount.