usually, leaders have a 90 or 100% to not attack if they are friendly. And all of them have this probability at 100% when friendly. This means that NO LEADER will decide to attack you at friendly, and most of them even at pleased. Some of them could do it (the 90% group)
However, this is about the AI deciding to go to war at a given moment. As far as I know, there are 3 situations where a pleased (with 100%) or friendly AI could declare you war:
- the decision to go to war is done before the declaration of war. This means that you could have an AI (with 100% at pleased) at cautious that decides to go to war with you, then for some reason she becomes pleased with you, and then declare war on you. Here an exception.
- an AI can be bribed to go to war. Some AI will never decide to go to war with you when they are pleased, but they may accept a bribe from another civ. And it appears that some civ will never declare to you at pleased, but are ok to go to war with pleased nations when bribed

And here is the trick about Cathy: she is the ONLY AI who will go to war with a friendly nation if you pay her enough (note that you have to be friendly with her to pay for that; so watch out for Cathy's friends, even if you are her friend)
- the apostolic palace can unleash a crusade on a civ; all the civ with the AP's religion will declare, independently of diplo status.