Yesterday i was playing a coop campagin with me as the Macedonians and my mate as Sparta. We then sieged Athens together with him on land and my units attacking from the sea, it all went very well to begin with, but at some point during the battle we suddenly weren't playing the same battle anymore, on my screen i had landed my units and captured one of the gates and my mates units were charging through that gate with all his forces.
However, on his screen my units were all dead having almost but not quite captured the same gate, and his units still standing outside the gates.

So we started playing the same battle, but ended up fighting different ones. Definitely one of the weirdest bugs i have ever experienced.