Here it is a CoA bug i came across:
Playing as Spain and i'm allied with The Papal States. France and The Papal States(also allied with France) are at war with England. Aragon(also allied with France) joined a coalition against me. I started a war against Aragon and called The Papal States to arms (yes you can) and guess what happened ? The Papal States didn't join the war.
Playing as Spain and i'm allied with The Papal States. France and The Papal States(also allied with France) are at war with England. Aragon(also allied with France) joined a coalition against me. I started a war against Aragon and called The Papal States to arms (yes you can) and guess what happened ? The Papal States didn't join the war.