When you try to purchase a unit that will take more than 2 turns to complete due to resources, the negative resource number never goes down (i.e if it says -26 the turn you order the unit, it still says that 4 turns later when it will be done next turn).
Aye, I had forgotten about that one. I will add it to the list, with perhaps odd things like "no more than fifty units recruited in a single turn" or "resources are directly linked to population, like unrest", though those are features rather.