one other thing that sometimes happens is a nation that you haven't met randomly declares war on you. imagine playing as, say Russia and out of nowhere: "MALI HAS DECLARED WAR ON YOU"
Sometimes, in a bout of Artificial Stupidity, an AI ally of vassal's army will do an annoying action known as "standing on your face" in which you'll have your army sitting in a province beneath the supply limit, but they tip you over so your whole army takes loads of attrition. it's only funny because their troops will wither away to nothing while yours are fine. Comedic Sociopathy I suppose.
another weird thing is when two nations with no army declare war on each other, causing said war to last until one of them builds something.
Chasing an enemy army isn't fun. add the benny hill theme. Instant funny.