It's hard to give small example with so general code, but in the end you either have very general flow (collision between any 2 objects can change state of the

It's hard to give small example with so general code, but in the end you either have very general flow (collision between any 2 objects can change state of the whole world, including the way the inputs are interpreted), that doesn't provide much protection against bugs, or you have very specific minimal flow (collision between 2 objects changes the state of these 2 objects, their parents, particle system, music system, and input interpretation, and nothing else) that protects against more bugs, but is a lot of work to maintain. I was trying to imagine a system which could give you the best of 2 worlds.