I already brought this up, but 1.20 patch doesn't seem to be taking care of it (the description I read of it), so:
Hidden Nationality land units bug. Although a HUMAN player can not attack and capture cities with them (unless declaring war), the AI can, and greatly abuses this. For me it's no problem, I can handle it, but the other AIs have no clue how to deal with it.
And another one:
Invisible units bug.
If your submarines happens to be in the direct path of the AI, he will attack and declare war, no matter what, instead of simply going round it. Same with invisible land units: if the AI happens to move a worker onto the square where you have an invisibe unit, it attacks and war is declared. He should just ask me to leave his territory instead. Again, the AI does not actively seek out my subs to attack, only if it's by accident in his direct chosen path, BAM, war !