There appears to be an Iron Bug in Civilization V: Brave New World, unless I am completely misunderstanding the iron mechanic. The mechanic as I understand it is that if you build units that rely on strategic resources, such as Iron, then lose that resource the unit performs at -50% capacity.
In my most recent game, as you can see in the screenshot above, I had 4 units of Iron and I was using 2. Nonetheless the game is reporting that “You are currently using more Iron than you have! All units which require it will fight much less effectively.” The thing is, I have 2 units to spare.
How did this bug happen? Well… I upgraded 2 units from Warriors to Kris Swordsman to Longswordsman. It appears that possibly the Kris Swordsman is the source of the bug. If this truly is a bug then I expect it will get fixed pretty fast.
