Had one recently.
Started up as Zapotec, managed to slap around the Mixtec and Totonac, but I had rivaled and laid a claim on a province against Kiche. They're allied with the Aztecs, who I am as well, but I think "No problem, I'll hit the Totonac one last time and then I'll DoW Kiche while the Aztecs are allied with me."
Yeah, I was trying to be super cheesy. Totally forget that in AoW the rules on that have changed a wee bit. Ended the war with the Totonac with my complete victory, head over to slap the Kiche down... and, months after the war started, the Aztecs break their alliance and join the war against me. Army gets sandwiched between the Kiche's "eh" stack and the doomstack lead by Monte "Freakin" Zuma. My army dies immediately, Kiche, Aztec, and some other ally of there's pours in.... and rage quit.
And it was my first Ironman "go" with the Zapotec. Everything was going great, had a 6/5/6 ruler in his 30s, and... it all ended. Peace would've costed me everything I'd earned and then some, not to mention my army was wiped off the map. After the rage quit, I had to try like six more times to have that level of success again. But now I know: don't piss off the Aztecs until Montezuma is dead and buried.