So I'm coining a new term: game math. It's math exclusively towards how gameplay works. Let me give you an example:

My gun does 10 damage. I'm shooting an enemy that has 50 health. It takes me 5 shots to kill it.

With normal math, if I upgrade my damage by 1, then my damage is now 11. This still means it takes me 5 shots to kill it. With game math, I've increased my damage by 0 because I'm not actually doing anything else. To elaborate, does the player feel like something has changed? If not, then your game math = 0

I discovered this with my latest game Dead Neighbors. I upgraded twice but it didn't seem to do anything. I assumed there was a mistake in my code and after exhausting tests, I discovered the above example. Now this extra damage will come in handy against larger enemies with more health but it can make your players feel like the game is broken. I ended up increasing the damage upgrade by 3x so you do +3 damage. This means that 4 shots is 52 damage. Of course that's overpowered as hell if I just left it, so the upgrade cost goes up by 10, instead of 5.

For any future devs out there, weird concepts like this will be a part of your life. I know it sounds so obvious and simple but when you start encountering issues like this yourself, trust me, it gets annoying.