Chariot Combat I promotion just before economical crackdown and STRIKE. I know, shouldn't happen, but some events (bot vassalization wars) made me make a whole 800-years war, which i, eh, lost.
I got the Chariot Combat one, myself. However, when I train units, I instinctively promote them with something (usually Combat I). So... all those chariots there really didn't get much of a bonus. Not the most meh, but pretty irritating.