I have noticed that, in oddly cases, when production automation is turned on in a city, the city governor uses gold to rush the building! I have attached a savegame. You can see that factory is 10 turns from completion in Hurrian, which has production automated turned on, and empire gold is 600. Next turn, the factory is finished and empire gold is 26. Turning off production automation in this city solves the problem in this particular case. Another cities with production automation turned on behave normally. A very odd bug indeed... Edit: Removed the savegame due to quota issues.