When loading a saved game one season's worth of Gildar and mana income from all cities is added. This bug is easy to reproduce: simply set tax rate to brutal, save the game and then reload it. Now your Gildar and mana stockpiles have both increased (this can be done ad infinitum for unlimited amounts of Gildar and mana). Resource (metal, crystal, horse and warg) stockpiles are not affected and neither is Gildar/mana income from heroes. Upkeep from armies is not deducted from the income gained.