I think this was a glitch in my last game... but I wanted to find out. Me and at least two other civs had entered the Industrial age. I had lots of allied City States. I kept trying to improve their coal, but it wouldn't let me. I go to aluminum, and I could improve their aluminum, but not coal. Then, I finally used a Great General to steal coal from a city state... I had my worker digging a mine... I forgot about it... and maybe 20 turns later, he still hadn't dug the mine. Then I clicked stop and mine again, and I had coal the next turn. Then, suddenly, all the city states that should have been giving me coal all the time finally did. It was very strange. That is strange as are the indestructible troops of my enemies. I hit a cavalry with four bombers, and it still wouldn't die. WTH?