Just noticed a rather good bug, at least for the human player anyway.

This was noticed for a modded game, but it could in theory affect the normal game too.

Basically, I modded a few techs to be untradeable, but I still got messages from the AI player asking me to trade them. The trade screen would show what they were offering, but my side would be blank. The dialog text shows they were asking for the tech I had modded to be untradeable.

Clicking accept would give me what they offered, and lo and behold a few turn later the AI would be back asking for the SAME tech, with another offer.