Every possible thing! We can't even enumerate every possible thing in chess with a compact set of rules let alone every possible thing in something like Red Dead Redemption with unbounded complexity. "Thing" is not "what happens when you shoot an enemy". "Thing" is "what happens when you shoot this particular enemy in this particular place at this particular in-game time and this particular not-in-game time after doing this particular long sequence of other very particular things".

That's what the issue tracker is for. If someone notices a subtle difference and they want it added they'll file an issue. If nobody notices the untested space it doesn't matter.