One that has hounded the players for a while I am sure. I am not entirely sure how to replicate it because sometimes I can do it and sometimes I can't but everyone who plays should know what it is.
basically you get something that stops you from using a certain menu, it fades out, and the only way to un-fade it is to move a unit, or cancel an order, etc
I believe that it has something to do with you moving a unit on top of a menu, as all menus fade out when you move a unit, you move it on top of it and sometimes it sticks in the faded position when you move the unit on top of it. but as I said, sometimes when i try to replicate it it happens, and sometimes it doesn't.
on and btw Amok and Ivan. I got that bug where you get submarines or destroyers travelling through land a few times. every time it has happened it has happened right at the end of my turn, on the last unit I tried to move. I think it has something to do with you releasing the mouse on the exact moment or maybe a few moments after the game sees it as the end of turn.