I'm not sure if this bug only exists if playing as Canada, but, you can pillage improvements/buildings with aircraft without needing to declare war (no pop-up). AI attitude does not change either. So you can really soften up a potential target before declaring war and they do nothing about it. I've noticed a few times when an AI spies siphon's funds, those funds end up in my total gold. I got a free 700 something gold from an English spy siphoning funds twice.