If I hit 'end turn', then I hit 'cancel', I often get a conquer screen (occupy, pillage, raze) if I conquered a city that turn, then I get an error because I already did that.
Sequence of events? As far as I can tell it just has to do with server connection. Although the overstack+crash causes this problem consistently for me. Try to move units through a full stack, and note that bug where your units disappear from the map, then refresh the page. Brings up the conquer thing again.
Next time it happens, dont press any button (or end your turn) just back out from the game and report the gameid here so I can have a look at the gamestatus... Thanks!