Kingdom: Classic - Bug - Free Upgrades

Alternative title: Getting used Coins back
This video illustrates a few examples, of how to use coins, without forfeiting them.
It's always the same principle.
Save & Quit the game after spending coins and before they vanish.
Reload the game.
The used coins and the bought tool/upgrade are both there.
This seems to work with almost anything, that needs coins.
Version: Kingdom: Classic 1.2.0 (R290)