1) Blood isle, monk in one of prison tombs. When you ask priest to open the room so that you could help the prisoner, intelligence persuasion doesn't work no matter how much int and persuasion you have. I guess it's broken. All other answers work fine.
2) Last tiger on Nameless isle. No matter what stats you have it's impossible to persuade him. People say that code of the game says that all persuasion is impossible. Bug?
There were couple more such situations but I don't remember em.
