In Super Robot Wars A Portable, there is a scenario boss that can join you later and when you manage to Ace him(get 50 kills and you get a unique ability for him) he gets ridiculously bulky the less health he has. That sounds helpful but that ability carries over back into his NPC Boss form in NG+ and I’ve heard it soft-locks the game.
Only way to avoid it is to know that information and don’t give him kills.
There was also this Star Trek Mirrorverse flash game where you could trade cargo with other neutral ships, usually each was selling something and would buy specific thing, but sometimes you could find a ship that would sell and buy the same thing, like you could buy ‘Stembolts’ at 15lat/ton and then sell it back to them at 25lat/ton, or atleast until they warped out.