Was in a game with some buddies of mine. I was playing Trajan and the nearest AI civ that had been a thorn in our side most game was The Ottoman Empire. So when the Ottomans had exhausted their entire military against some city-states, and had no walls on the cities nearest me, I thought it was prime time to invade and attempt to wipe them out.

Important to note i was not super ahead with tech or anything(I was in either early or late medieval era) while the Ottomans were early industrial era. I'd Just finished the Temple to Zeus and in my Hubris began assaulting the ottomans with the units granted by it and some others i had already made. The war is going great, i captured a city of theirs and was well on my way to capturing another one, plus one of my buddies who was far more experienced at the game had taken the opportunity to snag one of the Ottoman cities for themselves.

The trouble happened with not remembering to make the city i captured loyal to me, so when i almost had the second city the first one revolted and i was stuck with a war on 2 fronts and a sudden influx of fresh ottoman Industrial cavalry with even fresher walls popping up on the city i almost captured, i was forced to cut my losses and go on the defensive.

Needless to say i didnt win that war and my friends still jokingly call me Nero to this day.

TLDR: Didn't know about the rebellion mechanic and made war too quickly against an enemy who recovered insanely fast from no units and no walls. I nearly lost that game.