Easy to reproduce, I have done it over and over and over now. I apologize if this has been reported elsewhere already. I looked and didn't see it anywhere.
Emergent Gameplay - I was trying to see how high I could get my gold to go.
Conditions - I started a tiny map with 4 players. Defeated all the other players until there was just one AI left and that AI had only an army and no cities. I conquered all the cities from the AIs and kept them. The AI has a peasant, but doesn't have anywhere on the map to settle it, or at least they can't find such a place because they keep going around in circles in one small part of the map. None of my cities have anything left that they can possibly build. None of this probably matters.
Error - Research Refined Economics over and over and over while just passing turns. Gold income per turn counter goes up really slow. When I research Refined Economics this number doesn't increase that much. However, if I save the game and then immediately load the game, the bonus appears to get applied and my income per turn goes way up.
Income per turn actual values for demonstration purposes. Numbers are as shown when hovering over the gold icon with the mouse.
14,449.9
pass turn
14,781.8
pass turn
Refined Economics completes, select it again
14,939.8
pass turn
14,939.8
pass turn
14,939.8
pass turn
Refined Economics completes, select it again
15,097.8
save game
load game
17,967.7
The only thing different between those last two numbers was saving and reloading the game. Just for fun I thought I might save and reload without passing any turns and interestingly, the following happened:
save game
load game
17,994.5
save game
load game
18,021.3 (+26.8)
save game
load game
18,048.0 (+26.7)
save game
load game
18,074.8 (+26.8)
My current gold actually goes up every time I save and reload by some tiny amount causing my income per turn to increase by tiny increments every save/load cycle. All the increases were 26.75 per cycle.
Current gold values (not income per turn) while saving and reloading the game.
417,766
418,463 (+697)
419,161 (+698)
419,858 (+697)
I am keeping an eye on my research and it's not moving. It's not like turns are being passed and I don't know it. Somehow I am getting chunks of 697 gold added to my total without me having done anything other than saving and loading. This is hideously slow because I could just pass the turn and get +18,000 which is 26 saves and reloads worth of gold added, but it is noteworthy that I don't have to pass any turns to do this.
If you take my treaty income per turn of 1374.4 and subtract my unit wages (-20.9, turns into +20.9 with double negative) you get 1395.3 and if you divide that by two you get 697.65. The average increase per save/load is 697.5 based on the numbers I recorded. The numbers are perfectly correlated.
I loaded up a save I had with a different faction and went from 992 to 1022 after a save/load with no economic treaty in my possession. My city income in that game is 29.4 and unit wages are -6.5 for a net of +22.9 vs a current gold increase each save/load cycle of 30. Here my gold increases after each save/load cycle perfectly in line with my gross income per turn. I am pretty sure in this game that I have never researched Refined Economics and I am sure that I have not built the treasury vault.
I would hazard a guess that in my original game the +697 is somewhere around my gross income per turn without the treasury vault (and without Refined Economics?) and that the treaty income per turn is perhaps capped at double my regular income per turn?
