In terms of unit loss. Maybe some loaded, unescorted galleons running afoul of AI Frigates, or bringing a stack of 40 units and getting caught in open ground by AI siege or mounted units? Mine was an overseas invasion force that was strong enough to hold a city, but not strong enough to hold 2. I split my stack ater taking a coastal city and it was defeated piecemeal. Lost about 25 units and the remaining dozen or so retreated to my Transports. Lets hear your horror stories!