The first game that comes to mind for me is Civilizations 4. I’ve probably spent hundreds of hours playing but after getting used to 5 and 6 I have a really hard time going back. Going back and forth between 5 and 6 I need to rethink some strategies but with 4 I feel like I need to rethink everything. I don’t know if it’s because of the tech tree layout in combination with the civics system, things like unit stacking, or maybe just a bunch of little things but it takes me a while to readjust.
Unmodified unpatched original Morrowind had this strange bug where a goblin (can’t remember where, but he was in a castle) would sell you 5000 gold for 5000 gold. He would reset every day so you could continue this indefinitely. Then if you killed him you could then loot him for the accumulated gold you had sold him. (Let say you had done this 365 days it would net you 1 825 000 gold)